But, if one of your team fly out of the game for different reason (xbox crashs or some thing like this), he can rejoin the game.
The biggest problem with Joining an existing game will be the host change every time.
About the host problem: Why not letz one player open a server and the others join. Again, like in Unreal Tournament on PC.
Players can open a server and search the servers. That would be a much better system.