I seem to remember in Heroes III that the heroes added their statistics to the creatures under their command. Do they do that in Heroes IV?

Basically, what do Heroes actually do in Heroes IV?

They also join in the fighting directly.

Primarily I'm wondering if my Hero's attack and defence are added to the attack and defence of said hero's creatures? What if I have 2 heroes in a group? Do they both affect the attack and defence of my creatures? Do my heroes affect each other?

Ok, so HOMM IV is unlike the previous Heroes games where the attack and defence of the hero are added to the creatures under their command? The only time a Hero adds to it's creatures are when it has the tactics skill and then all that is added is a percentage based on the skill level of the hero, not what the actual stats of the hero are?

Exactly.
Also there are a few artifacts, which add to your creatures' attack or defense, like the Tactics skill.

Warlord's ring and Crusader's mace add +20% and +30% respectively to melee and ranged attack, Ring of protection and Steadfast shield add +20% and +30% to melee and ranged defense.
I don't remember how (if) it stacks with tactics, but these artifacts are quite common, so you won't need to get Tactics at all costs.
Also a few items has another beneficial properities for your units:
Sniper's crossbow removes your ranged units' range and obstacle penalty
Spear of the Centaur lets your ranged units have the "No melee penalty" ability
Five shields affects your units with the <allignment> Ward spell
Dragon scale shield gives your units Fire resistance
etc.

Thanks, this has been a big help in understanding the game!

As I said above, "the highest bonus applies". So if you have a Grandmaster Offsense (+50% attack) on one hero, Master Offense (+40%) and Warlord's Ring (+20%) on another, and Expert Offense (+30%), two Warlord's Rings (+20% each) and a Crusader's Mace (+30%) on a third, a bonus of 100% (30+2*20+30) attack will apply to each creature. And yes, that is a percentage of the creature's base attack, not hero's. Hero's attack rating applies to that hero's combat rating on battlefield, not to his creatures.