When you first link your card or create an account via the app, you will have a choice of teams. Look in the drop down list to find a type of team followed by the name of the team. Players can join any team, and everyone is invited! Get all of your friends and family involved, including brothers, sisters and grandparents.
If you don't join a team when you link your card, you can do this by going to "Manage Account" via the menu and selecting your team.
If using the app, visit Account and tap the blue bubble with your game name to change your team.