Alliance members can see details about their alliance (on the Alliance screen).
They can send resources or troops to allied cities (go to the allied city on the map and click on the city).
A city can host as many allied armies as the level of their embassy.
They also have their own chat room and forum to discuss alliance issues and plan their attacks.
The Chancellor of an alliance can designate other alliances as “friendly” or “hostile.”
Officers can also invite people to join their alliance and/or kick people out of their alliance.