Challenge Alliance Process

When you click to challenge some alliance, you get to pick a map first. When you confirm it, they are notified about your challenge. Once their leader accepts the challenge, game starts and players from both alliances can join.

Thanks, now what about if new members join my alliance and I already have a challenge going on. Do they have the option to join in or must they wait until I create/accept another challenge?

Once alliance challenge is accepted, all your member have the option to join it (if there are free slots left). So, be advised that some people you don't want in the game can enter it. Best thing to do about it is to either password protect it or tell your people not to join it.