You can set a restriction on how Players or other members can join your Team. This is important so you can have control to those who would want to be a part of your Team. You can do that by: 

1. Clicking on Settings on the League Page. 
2. Click Team Privacy.
3. Set Who can Join the Team, Who can Become a Fan and Who can Invite Fans. 
User-added image
You can set the following options to Anyone Can Join, Team Controlled, Admin Approval Needed and Invitation Only.