Re: Days of Future Past News and Discussion - Part 10
They're team members at the end of X2. Colossus and Kitty were cameos in X2 that got promoted in between X2-3... Colossus probably either proved his worth in the mansion invasion or realized his purpose as part of the team at that point. Kitty who knows how she joined. Maybe she always wanted to or became friends with the gang and told her to join up...
It's not like it's some official contract they sign or something... It would seem like the X-Men take who they can get. Wolverine became a team member after waking up in the mansion and stealing vehicles and insulting everyone.