Organic Groups Membership Types
If you go to Configuration > OG membership types, you can create various membership types.
For example you can create a membership type that expires after 30 days. This would enable users to test your group for a while, then be removed from the group after the 30 day trial is up.
Check out the thread at http://drupal.org/node/1233888 for updates on this functionality in Drupal 7. In that thread you will also find a sandbox project that can get you going right now.