Meetup.com makes it a pain to unsubscribe from groups. You have to click on each group, wait for the page to load, then click on the unsubscribe link, wait for the page to load, then click the unsubscribe link. Doesn't sound too horrible but when you have a bunch of groups it's pretty tedious. I can only assume that Meetup doesn't provide the functionality to do this because you being a member of a lot of groups is good for their metrics.