[nokya]

Reselling (and sometimes even worse: handing them over for free) abandoned groups has been part of Meetup's original business model (or at least for a very long time). I personally avoided this bad situation only by luck, thanks to a friend who had her company group and all members handed over to a competitor.

I am really sorry you were tricked by this. It is documented in their terms of use (which nobody reads) so I don't think there is much to do but to spread the word and warn other Meetup users around you. Let them know to make sure they get everyone to leave their group before they leave it or abandon it.