When searching for opponents, the game picks the league for the group members (up to 4 players) according to the league of the player who has the HIGHEST league placing.
*Note: Maximum difference of level in group can not be higher than 15 levels. Maximum difference of league level in group can not be higher than 2 stages.
What if my mate is in the different league?
If you are from Gold 1 and your teammate in squad is from Silver 2, you will be matched with the opponents from Gold 1. But you will earn and/or lose rating depending on YOUR OWN league.