Meetup is a site and tool that allows you to create a group and organize real life meetups (remember those?). Usually these were location based (like Miami Movie Makers) so if you had a company or movement, you couldn’t really enable independent groups to organize and meet.
You’re probably already one step ahead (and are astute and read the headline). Meetup Everywhere could be a great way to determine where there’s an audience that wants to see your film.
When you list what you’re Meetups are about (your film) you could put something in the description like “Meetups with over 150 interested will get a screening,” or something to that effect.
I’m not sure if this has PayPal integration like the regular Meetup does for member dues, because if so you could self enable smaller groups that you couldn’t fly out to to have their own screening and have them buy tickets online.
There’s Eventful, which seems to do a similar thing. But I feel Meetup is really well established, and has a cleaner and simpler interface.
(I could have sworn there was a film specific site that did the same thing, but can’t find it anywhere. Does anyone remember the name?)