BuddyPress.org

You could probably hook into the get_header action, do your conditional check and if the person is a mentor, then show the page, otherwise show the registration page or whatever.

Modify the function listed in this thread:
https://buddypress.org/community/groups/how-to-and-troubleshooting/forum/topic/how-to-make-a-private-community/?topic_page=2&num=15#post-44616

Hopefully it gives you a few pointers.