When someone has registered for an event you manage, and accidentally registered as an individual instead of joining their team, you can manage their registration to move them to their team.
To do this, go to 'Manage registration' on the left hand menu of your Event Dashboard. There you will see a list of everyone who has registered for your event.
By clicking the 'manage' button next to the relevant participant, you will open their registration profile, where you will see multiple buttons on the right of the screen. To move them to a team, click 'join or create team' and follow the steps.