Sorry for the wait, got some more info on this for you:
Teams are assigned by the game in order to keep the pace of the game-flow up. The game will try to keep Club members in the same team, so if you have a session comprised of two Clubs the teams will be split along Club lines. Scoring for team races is actually designed to allow uneven teams to battle fairly (e.g. a 3 man team can beat a 6 man team if they are better drivers) so even if you dont have even numbers your Club may still be kept together in one team. However, we dont allow (say) 11 Club members to go in one team against a lone player in the other in instances like that, the Club will end up split between the two teams. However, you are still earning Club XP bonuses regardless, which benefits you and your Club.
Hope that helps?