If you enter the match between rounds when it says like round win or switching sides, often times it lets you decide which team to be on. If you have a party you tell each other to go on the same team. Or you just press select and see which team has the most people already and join that team. Once my team had 9 and the other had 3. We had a three person party as it was 6 v 3 when we were entering but instead of going with the 3 like it wanted us too, we went with the six.