As the event administrator, you are able to add someone to a team or remove them.
Log in to your account and on the Home tab, select the event you wish to work with.
Select View Participants.
Choose the fundraiser that you want to work with.
If someone is not yet a part of team, you can add them by clicking "Add Team" and selecting the appropriate team.
If your fundraiser is already on a team, you can remove them, or simply change teams altogether.