By default meetup invites include an ical attachment. Mail automatically pushes this file into your Calender's Incoming folder. Calender then reads these files and creates the event.
I noticed in my incoming folder (had not started Cal for some time) that there were 100's of calender ics files.
Subsequently you can tell meetup not to send you invite email for the groups. There is a way to stop meetup from sending the attachments but I have not figured it out.
the_lemmingukis right -- about the .ics being pushed into iCal by Mail. If you do a search for files with ".ics" in their name, you'll find them as attachments in:
[username] > Library > Mail
[username] > Library > Mail Downloads
The name of each .ics will be the name of the event -- and you can view the data in the .ics with a text-editor like TextEdit. You might want to do so, if like me, an event date has changed since the .ics file got pushed into iCal, and because it's a meetup event, iCal won't let you edit or even delete the event -- muy annoying!
Anyway, I have a meetup event whose date has changed, so now the event appears in my iCal twice: once for the earlier and incorrect date, and again for the recently updated and correct event. Opening all of the .ics files with the event name as their file name, I look for the variable:
which is the start date of the event in in the format: [country]/[city]:yyyymmddThhmm?
y, m, and d are digits, and T indicates the beginning of the time-of-day field. I don't know what the last digit, the ?, stands for, but whatever it is, it's not important as all I care about is deleting the iCal event with the wrong date.
Delete the .ics file with the wrong date, and this will delete the event from iCal after iCal is restarted.
You can log into the MeetUp site, go to your account preferences, click on email and notification settings, scroll down to "Other MeetUp Email," under that heading is the option to have your email notifications send an iCal attachment. If that box is checked, your calendar will automatically add MeetUp events to your calendar regardless of whether you open the emails or not. Simply uncheck the box.
Hope this helps!!