Re: BuddyPress Membership Sites
I have a wishlist member plugin license that I am planning on using on a wpmu site. When I contacted wishlist asking if that plugin was compatible, they had told me that it wasn’t tested but they thought it would work just fine.
I can give you more concrete feedback in a week or so.
Other than this, unless you are interested in restricting BP-specific pages to different membership levels, you are really looking for a wpmu solution. This wpmu-dev plugin claims to do that.