Allied matches require three or four players in the same match. To choose alliances once you are in a lobby, press on your gamertag and pick a color at the Player Alliance field (you will be on this field by default, and pressing will cycle through the available colors/teams.) Ensure that you have two players on the winning team. This is possible to complete quickly with three players in a private match online by having the solo player surrender. To speed up the games, ensure that Random Map is set to ON and Teleport In is set to OFF.