I see this game has co-op, but what exactly are the details? I've searched for a few days and I can't even get a positive answer on how many players can play together in whatever type co-op game mode(s) it has: Is it built into the game? How many people can play? Is it the complete campaign or something separate? Is the co-op referring to a mod (the multiarena one)?

If you don't know the answers, I guess all I really need to know is if I buy this game now, will I be able to play the full campaign co-op with more than one other person?

http://www.terra-arcanum.com/sierra/game/game-multiplayer.html

Everything about the multiplayer/coop mode.

Basically turn based combat is disabled and only real time combat can be played. Fast travel is disabled but maps are smaller.
Didnt read anything about player limits though.

I just bought the game too you can send me a PM if you buy it and need a partner to try the coop mode

Arcanum is one of my all-time favorite games - I still have my big box copy, purchased the week it came out without having a clue what it was but just thinking it looked really cool - and I somehow had no idea there was a co-op mode. Weird!

Right. Terra-Arcanum will not answer any of your questions.

For real answers I would refer you to my own topic,

http://www.gog.com/en/forum/arcanum_of_steamworks_and_magick_obscura/arcanum_co_op_engine_conversions_and_more

But to shorten that if you're not interested in reading it, your best bet for the single-player campaign as a co-op, realtime experience would be the WIP 6.0 co-op mode. There are two big hurdles still, saving and getting past the BMC. From what I understand, the "map" is massive, with all the areas of the game up until the BMC as one area and you 'teleport' between them. For whatever reason, the BMC mines wreck everything.

Otherwise the best you can do is get modules, like the built-in Vormantown one. Unfortunately, modules never caught on because the game never did so there are pathetic few from which to pick.

Does it work with IP?