Hello Damerei,
So far there is no way to choose your game, teammates, opponents or mission. A while back Whiplash organized a tournament to pair up opponents. It had limited success partially because it required the cooperation of everyone who was playing. Once the players finally met in a "private corner" in the right kind of mission, other players could end it by having the nerve to win a game.
The ability to choose sides and missions would be a great addition to the game. Someday....?