minimeetup.com - <some event>
The plan is to meet at <some time> and <do some stuff>. It will be <really fun>.
3 people have RSVPed:
2 other people have said they might be interested if there are this many people, but haven't RSVPed yet.
With 1 more RSVP, a further 3 people have said they might be interested.
With 5 more RSVPs, a further 1 person has said they might be interested.
"I want to RSVP":
<name> <email address>
[Sign me up]
"I might be interested if you get a few more people":
<name> <email address>
[Submit]
John Smith
Peter Ludd
Joan Clockson
* I might be interested when more than <a number> of people have RSVPed
OR
* I might be interested when more than <a number> of people also might be interested.
Website could be very minimal. No accounts or passwords or anything. Each new event is just allocated a random URL, and is not accessible unless you know the URL. Anyone can create a new event by filling in a very simple form.