Meetup, the drum circle listings are good places to start. Google your area and "drum circles". When I was in LA (and still do) I would take group drumming classes and then meet other people in the class. We would then organize/ publicize a drum circle on a particular day (second sunday of the month) and if noboby showed up we would just practice what we learned in class. If people did show up we had enough rhythms down that it provided a solid foundation for whoever came to play.

If people know the drumming is solid they will come back and spread the word. It also helps to have a table and call it a potluck or whatever too. I prefer no alcohol around the circle, and I am a drinker. So maybe decide how you want the ideal vibe to be and do things to model that from day one. Make a few flyers for next months jam, so people have the impression its a solid consistent thing they can plan on. Get a website or email list or start a (your county/area)drumcirclelist@yahoogroups.com

good luck

Ahh yes 4 Quarters... I know it well. Wifey and I were members there in 2001. Thanks for the link.