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/