This is really bad that this can happen.
I've also been told that it's tricky to remove a group, which means if you want to disband it, you have to manually remove all your members one by one unless you want to potentially subject them to a spammy takeover like has happened here. Removing members from your groups is also a pretty painfully slow process.
(edit to answer my own question:) I found this & yes that seems scary as hell:
"Neglect to pay the Org dues for greater than 14 days, in which case the group becomes available and anyone can take it over"
Given the way the site looks and functions, I'm surprised there isn't a more popular alternative. If anyone knows of any, I'd love to hear about it.
From the http://www.meetup.com/terms/
3.3 Meetup Group Fees;
[...](3) Meetup Groups. Organizers who allow their Basic Group Fee Subscription to lapse are subject to removal as the Organizer of the Meetup Group that they lead, and their Meetup Group may be terminated or transferred to another Organizer. [...]