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 don’t have even numbers your Club may still be kept together in one team. However, we don’t 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.
So, there you have it, if you really want to win a match in team then you need to have skills to do so, otherwise your team mates will not be able to help you in any way.
Forza Horizon 2 is set to launch on 30th September on Xbox One and Xbox 360.