BuddyPress.org

You could always just use htpasswd to compel http authentication too. It’s a bit of a pain for users to have to log in to both the directory, and into buddypress, but it will give you the protection you need. Or you could just stick something into the theme to say that if the user isn’t logged in, they’re always redirected to the login / register page. I need something similar, so if I make any progress, I’ll post back here with it.