I put out a couple of blogs about the SciFi and Fantasy group and it got reblogged a couple of times, plus someone blogged about our group read, and that got reblogged, so we had a lot of exposure which led to a membership boom on the first day of discussion rooms. I'd guess that some Romance or YA groups will eventually have the largest membership.
The grubs are linking to this room and giving it free advertising. Go grubs!