Connects people around topics of interest allows members to find and join groups unified by a common interest. Remarkably inexpensive to run - about £20 a month to run your own online group that you can keep private.
Sussex Gay Professionals...
http://gaypros.meetup.com/183/ London Bisexuals...
http://lgbtfriends.meetup.com/145/ Brighton Lesbian & Bisexual Womans Arts Appreciation ...
http://culturecircle.meetup.com/27/ Transgender...
http://familiesoftg.meetup.com/cities/gb/london/