To hide your Meetup Groups, make sure you're logged into your Meetup account, and then click on 'My Groups' in the top right. You'll see a list of your Meetup Groups.
Scroll up just a bit, and you'll see one of two things beneath the 'Member of # of Meetups' heading:
If your Groups aren't hidden, you'll see a link to 'Hide my Meetup Groups from others'.
If they're already hidden, you'll see a link to 'Unhide my Meetup Groups'.
If your Meetup Groups are hidden, the list of Groups you're in will not show up when other folks look at your profile in a Meetup Group.