I'd like to add this about cooperative combat:
Rallies: Rallies use the talents and tech for the initiating player only! DO NOT just begin a rally on a whim and get all your friends' troops killed. Be responsible with that trust. Tool for war, and communicate with your buddies how you are geared. If you tool for Cavalry/Ranged, be sure to say that, or else you may have someone with infantry get upset that they suffered heavier than normal losses.
Reinforcement: As with rallies, it will use the talents and tech of the host player. Be wise when reinforcing, and be considerate when asking for reinforcement!
Garisson: Unlike the others, Garisson size isn't limited by host player, and the contributing player CAN send their heroes. The downside is that only one player can garisson, and the garisson battle is completed SEPARATE and FIRST.