As one organizer for the Mid-Atlantic APUG group, I can say we've had success by doing two things: One, getting the word out, and Two, being dogged about scheduling an event and sticking to it. I think it helps alot by having the nature of the event and a firm schedule well decided in advance - most people are joiners, not leaders, and will be glad to show up for the beer and pretzels, but if you offer too many choices, nobody will come.