@David Payne – The buddypress rules are still a bit, well, weak – I’ve got some time scheduled to sit and go through buddypress and add in all the rules I can. At the moment though there is only so much I can restrict from the membership plugin as a lot of the buddypress functionality (such as the showing or not of the “Join group” button) just doesn’t have a filter or action that I can hook into and override.
I’m adding an increasing number of helper functions to tap into the membership plugin functionality, but it’s looking very much like some of the more advanced functionality and restrictions on the buddypress side of things are going to need edits to the bp themes.