Most successful sites have a user interface / experience tailored specifically to their user’s needs; with a compulsive reason to be there. Just putting up a generic platform with groups, activity and members directories, and hoping they’ll join is unlikely to work.
BuddyPress gives you a great platform to underpin your build, but you will need specialist design and development skills to craft that compulsive site with features your users will want and need to use.
[ Edit: for another large membership BP site, take a look at http://enterprisenation.com ]
