I am not sure why suddenly Meetup decided that they would charge a fee for people to use their service, but it seems to be very successfully driving people away … and to Yahoo Groups which offers very similar features (minus RSVP options for events) at no charge.
As soon as the administrators of the groups I am in received notice of the change, they announced matters to the group, who all support the migration.
If I was Meetup, I’d start to reconsider things. The discussions have taken place within the Meetup discussion boards… we’ll see if they are paying attention.