I send the invite out to the Meetup group first, so people who are a part of the group get "first dibs." Then I post to HN, and then I send a last message out to the group to catch anyone who missed the first email/forgot/put it off.
There were about 90 people RSVP'd before I posted this thread up.
So if you want to be sure you get a spot next time, join the group and look out for the announcement email! :)
http://www.meetup.com/find/?keywords=Hacker+News&mcId=z9...