Rob brings up a good point. Meetup recently changed the way that the waitlist works. The old way was that if a spot opened up you would get an email that there was a spot available and you would have to physically go to the home page and change you RSVP...it was on a first come first serve basis...the new way is as follows:
If a member is first in line on the waitlist, they'll be automatically RSVPd to the next spot that opens up. The only exception is if the member on the waitlist wants to bring guests, that member and their guests will be skipped over until enough spots open up at the same time for them all.
So if you are on the waitlist for an event, please keep an eye on the home page to see if you automatically get RSVPed into an open spot.