Turn on Dynamic teams by going to Event setup and Distances and prices.
Go to Settings and Registration type must be set to Dynamic team registration
Number of participants on a dynamic team can't be specified since people can join and leave teams as they wish.
Sign up example
When activated, participants can decide to not join a team, join an existing team or create a new team.
Participants can also join and create new teams using their Participant edit code.
See also: Team results