When you're invited to perform at the Crew Battles event, you're sent an email with a form you need to fill out with the name of the Team you want to join. Artists are encouraged to communicate with each other and split up between different Teams to compete, filling out this form when you know what is the name of the Team you will join. The deadline date on when it should be sent will also be listed in the form itself.
If you failed to fill out this form in time, you'll still be able to perform and be featured at this event - however, you'll not be assigned a Team and will not qualify to receive any Team-related rewards, only being eligible for individual rewards. To avoid this situation, make sure to always fill out the form sent to you when registering for the Crew Battles Sessions Event.