Ask HN: How do you monetise a group meetup

I've got a group that regularly meets in bars, restaurants, theatres, cafes which is organised through meetup.com They're pretty switched on and I can easily get 60+ folk along to a meetup at any one time.

I'd love to turn this in to a business but have no idea how to monetize? From the user side or business side? Offering businesses the same benefit as a daily deal site could work, or ask for kickbacks, but it seems like a lot of business convincing (read sales) Membership dues doesn't sit right with me as the appeal is that many events are free to attend.

Thoughts?