Arcanum has many potential allies in it for a would-be hero. Some are good, some are bad, and some are really just not that important. Many will die horrible deaths fighting for you because, frankly, their stats suck and their AI consists of a digital cabbage inside their poorly-rendered heads.
I have taken the time to write abilities, where to find them, and how to get them to join you, because surviving the trials of Arcanum alone is murderous, and not all of these folks are easy to discover. Here they are listed, roughly in the order that you can find them. Note that characters who only join you temporarily as part of their own sidequest are not listed here.
Your maximum number of followers is dependent on your charisma. You're allowed one follower for every 4 points of Charisma. A Charisma score of 8 allows 2 followers. A CHA score of 20 allows 5 followers, and also grants you the ability to ensure that no party member will leave without your permission. There are some backgrounds that can affect this. For example, Educator takes away one follower in return for other bonuses. However, gaining the Expert level in Persuasion adds one follower. Masters of persuasion can talk people who normally wouldn't tolerate each other into staying with the party.
Virgil: That dude you meet when the blimp crashes. Don't worry about finding him, he'll find you. Besides serving as a source of exposition, Virgil is a competent fighter and healer, and one of the only characters with full voice-acting. First-time players are advised to keep him around for the entire duration of the game. If you let him, he will join you and fight by your side no matter what.
Sogg Mead Mug: An ogre you can find in the bar in Shrouded Hills. Not much of a character, but like all half-ogres he is massively strong and tough, an excellent melee fighter. Getting armor in his size might be a challenge, but if you can get him early in the game you'll have a substantial tactical advantage over anybody who does not also have an ogre on their side. Getting him to come with you requires a charisma score of at least 9.
Jayna Stiles: A chemist from Dernholme. She will only join you if you do not have a substantial magical alignment; you must be bent towards technology or nuetral at the time you ask for her to become a party member. Although she might seem like an interesting person before she joins the party, all of her sparkle will fade as soon as she becomes a member. She's also a poor combatant and unable to equip heavier gear. BUT, she is very useful in one respect: she can mix potions for you if you can get the ingredients for her. Given that the plants needed to brew restorative brews are pretty common, keeping Jayna in the party is a good way to get access to cheap healing. Be warned; Jayna will not tolerate you killing good people.
Vollinger: A suspicious-looking halfling from Dernholme. Vollinger is a passable fighter, and like Jayna he can use his chemistry knowledge for your benefit. He can also make guns. However, you cannot have both him and Virgil in the party at once unless you max out persuade... and by the time you're a high-enough level to do that, Vollinger will be useless to you.
Magnus: A Dwarf waiting outside of P. Schuyler and Sons in Tarant. He is a decent melee fighter, but if you want to play an evil character he'll soon grow angry at your civilian-massacring ways and leave the party. He's easy enough to convince to join, though, as long as you're polite. He's got more personality than most allies, but his most important trait is his smithing skill. He can make pure ore and the game-breakingly powerful Balanced Sword for you if you bring him the materials.
Dog: From Gamebanshee: "You’ll find Dog being attacked by a gnome the first time you see him. If you rescue him from the gnome, then Dog will join your party and not count against your follower limit. Since Dog will also earn master training in melee, he’s one of the best followers in the game.
Note: It’s not clear when the gnome starts kicking Dog. So if you want to add Dog to your party, you should head for him right away when you visit Ashbury for the first time. Otherwise the gnome might kill him. "
Geoffrey Tarellond-Ashe: A vainglorious necromancer from Ashbury. He will only join you if you're evil. Spellcaster AI in this game is sub-par, so I don't recommend seeking him out; he'll blow all his spells on the first enemies you encounter and then get himself killed trying to knife the foe to death.
Torian Kel: The ghost of Kel "lives" in a place called the Ancient Temple, and a long sidequest must be undertaken to bring him back to life. You must be evil to get him in the party, but it's worth doing his quest just because he knows more about the game's backstory than almost anyone else in the game, and will tell you all of it even if he doesn't join you. He's a capable melee fighter.
Chukka: Gilbert Bate's manservant/bodyguard may join you after you've done a few of Gil's jobs. See 'Sogg Mead Mug' for details on why ogre fighters are kickass. Bates is, of course, in Tarant.
Gar: The world's smartest orc. If you have 2 ranks in persuasion when you first meet him, you can ask him about tea to make him reveal his origin. You can then blackmail his owner into giving him to you. He can be a brawler for you, but I'd recommend releasing him to gain a fate point and pick up someone else. The museum Gar resides in is in Tarant.
Loghaire Thunder StoneDwarf King. Getting him into the party is a challenge that requires high intelligence and persuasion. Talk to his son Randver about the Stone and Shape, and then talk to Loghaire about the same. He is a good meleer, and works well with Magnus on a certain dwarven history sidequest...
Jormund: Dwarf wizard in Qintarra. Will join you if you clear him of a murder charge. Doesn't seem very useful, because he's lousy even for a spellcaster, but do his quest anyway for the XP and his magic staff.
Waroman: Big lizard from the Bedokaan village. To get Waroman to join your party, you must resolve the conflict with the poachers through dialogue. Then, if you tell the Bedokaan chieftain Kan Kerai your story, he’ll suggest you take Waroman with you to help you out. Waroman will eventually gain expert training with the bow, and master training with melee weapons, so he’s a good companion to have if you’re diplomatic enough to get him.
Perriman Smythe: Another one of those adventurers you read about in the papers. Smythe shows up in Tarant after you visit the Elf Village. He's a powerful sorcerer, which means that he'll waste all his magic and then turn into a water elemental for no good reason. Don't bother with him unless you think your party really needs a healer.
Sebastien: Sebastian will only show up after you’ve spoken to Edward Willoughsby about the “personal measures” he’s taking in the Boil in Tarant (Ed, like Perriman, doesn't show up until you go to the Elf city), and killed both Boil gangleaders. He's a passable fighter and carries some good equipment when you first meet him. He can also make explosives and electrical devices for you. He's easy to overlook, though, because of the very specific requirements you must meet in order to get him.
Z'an Al'urin Dark elf from the Dark Elf village. She will join you regardless of your alignment if you're completely honest with her. She's a capable spellcaster, but that doesn't count for much in an NPC. She does, however, get to have an interesting conversation with Arronax, your archnemesis. Only bring her along if you're really addicted to story elements.
Raven: Elf princess. Incompatible with Z'an unless you max out Persuade. She's a bow-user who can fight passably well as long as she's supplied with arrows, but I'd recommend not bringing her along because there are others who can do better.
Franklin Payne: Payne will only show up in his home after you’ve visited the dark elf village, but before you go on to the next story location. He's a great storyteller, a cheerful adventurer, and very, very British. Good with guns and melee weapons, and recommended for both reasons. Good voice acting too, which only makes the stories he has to tell of his adventures that much more fun.
Weldo Rubin: Halfling hero who will meet you at the entrance to the Wastes. Not a bad character, but sadly underleveled for the time you meet him.
Prisoners of the Void: Besides the ultimate villain himself, most of the named (non-slave) characters you meet in the Void can join you, and they're all quite powerful in their own ways. If for any reason you don't already have a full party by the time you reach the endgame, feel free to recruit some or all of them. I leave it to you to figure out how.

Your guide is much appreciated !
I have been looking for something like this.
Could you please add, how the number of followers is calculated exactly ? I know charisma and persuation are important, but I dont know how.
Many thanks

Your maximum number of followers is dependent on your charisma. You're allowed one follower for every 4 points of Charisma. Charsima of 8 allows 2 followers. There are some backgrounds that can affect this. For example, Educator takes away one follower in return for other bonuses. However, gaining the Expert level in Persuasion adds one follower.
Prator, you forgot
Jormund: Dwarf wizard in Qintarra. Will join you if you clear him of a murder charge. Doesn't seem very useful, but do his quest anyway for the XP and his magic staff.

Oh yeah, that guy. Right. I'll add him to the list.

I don't think that last part is true. Maybe because I had 19 Charisma and Expert Persuasion, but I had both in my party by level 25ish.
BTW, my understanding is that you can get 20 Charisma, and therefore have 6 followers max. Is that true/how?

What level was Vollinger when he joined you? These NPCs don't scale to your level, after all, and after a certain point most of them will be prone to dying a lot against the monsters you and your leveled-up friends don't have too big an issue with, even if you give them the best gear you can buy.
How high your charisma (or any other stat) can get is dependent on your race and whatever other modifiers were applied to your character by your background. I think you're right, 20 is the maximum charisma score for a someone with no penalties. However, 20/4 = 5, not 6. It's the Expert Persuasion bonus that nets you the 6th person. You can get up to 7 if you count the Dog. As a sidenote, getting a CHA score of 20 gives you an extra superpower. I quote the manual:
100% loyalty: followers will never flee from your side and will only leave you if you ask them to, never of their own accord

How are levels calculated for when you pick up a character? Do they level every time you do? I picked up Sogg when I was lvl 14 and he's lvl 2. I haven't levelled up since then so haven't been able to test it.
Is there a convenient way to level them up if the above is not the case?

NPC stats are predetermined; the stats they have when you first meet them are the stats they have when they join the party. As for XP, I'll let the manual do the talking:
"Followers do not deprive the player of Experience Points, with the exception of the experience per blow in Combat which the player does not receive. Anything killed by a Follower is treated as a kill by the player's Character, for purposes of both Experience award and for any Alignment shift that it may cause. Additionally, when the Player Character goes up in Level, each and every Follower of that Character will also go up in Level. Followers will spend their Character Points according to an Auto-Leveling Scheme of their own devising. "
Thus, the fastest way to level your allies is to level yourself, and that means exploiting the XP-per-blow as hard as you can.

On the topic of party NPCs, is there a mod that grants you control of them in combat at all? I mean, Virgil for the love of buddha please heal Magnus when he's near death, that's all I'm asking!

While I don't know about mods, Virgil will heal you, himself, and others more or less in that order. If you're really banged up, he'll spend all his mana getting you healed to the brim. If you manage to come through without a scratch, he'll heal himself, and if he has any mana left after that, everybody else. At first, Virgil's magic can heal Magnus some, but as Magnus gets more techy, Virgil's spells work less and less well. So check Prator's NPC list above and a) go get Jayna to heal Magnus (if you have enough Charisma to take another follower); b) stock up on techno healing salves by purchase or theft; and/or c) take a point into techno healing to be able to make your own salves out of plants you find.

Oh that's right! Thanks Luned, I completely forgot that Virgil would be ineffective at healing Magnus with the Techno/magick relationship. Thanks!

Imma gonna bump this...

Just found the hard way that Virgil will leave the party at a certain point later in the game, even though we had no disagreements. There wasn't an option to talk him out of it and I'm not certain that there will be an opportunity to pick him back up again. If you stick with Virgil make sure you have at least some ability to heal yourself (cures for poison are good, too) if he leaves for any reason.
Edit: Ahh, looks like Luned was kinda saying the same thing, just for a different reason.

Thanks for the spoiler. I was just wondering last night if there was any good reason to visit some of those "early" towns again, short of needing master training for this or that, since the story hasn't sent me back to Dernholm or Black Root in quite a while.