As on their website:
Members will pay a $2 fee when they reserve a spot at your event.
So if your 200 attendees RSVP-ing to your FREE meetup you as the organiser you will be charged $400 per meetup.
You can read angry reactions on twitter #MeetUp as well.
I think in the future you’ll see these messages as a solution in several (or in all) meetup groups: