You can allow your Team Captains to set a password that is needed to join the team.  To do this:
  1. Go to  Fundraising > TeamRaiser
  2. Click the Edit link for the TeamRaiser you wish to change.  
  3. On '4.Select Team Options' look for the option titled ' Password-Protected Teams'.  
  4. Check the box that says ' Yes, allow Team Captains to assign passwords for admission to their teams'  Step 4.