There are like three ways to add coop
1. Time runs the same in cities and in battles. While any of the player is in a city or fighting a battle all other player's speed slows down to this minimum. While all the players are on the strategic map the time speed flow can be adjusted by a server admin/ane player (the most preferable option for me)
2. While any player is in a ciity or in a fight other players are forced to spectate or, in case of a fight, join the first player's side (or fight against him) as one of the attackers/defenders (play as a regular soldier in an army)
3. Only one player commands the army. Other players are always traveling with him. (The least preferable option for me but still an awesome gameplay)