I was trying to play a private match with my clan, but we could not get more than 9 players in the match at once because the Xbox Live lobby would only allow 9 players. Then we were not able to add anyone while in the private match lobby. Does anyone know how to play with 10 or more players? We all remember doing it once, however, no one could figure it out.
Have you tried adding them to the game without having them in an Xbox LIVE Party? Some gamemodes don't allow large party sizes due to the team size.
The only way i can think of playing with more than nine friends in a game would be for them to join your game after it has started.