How will you know?
Organize people, get them to commit to a date, time and place. Use software to get their commitment one way or another. If people change their mind or something comes up, they can cancel and people attending will know not to expect them.
Notes: There was a #Breakfastup tweetup (meeting of Twitter users) planned for this morning, Friday March 18, 2011 for 9 AM, and there were a bunch of cancelations and no shows. There was no way to know who would show up. Actually, it would have been good to see who still planned on going, it would have enabled the event to be rescheduled. In the end, only three people attended including me. We did have a good conversation.