I'm one of the organizers of our Bay Area R Users Group (BARUG). See
http://www.meetup.com/R-Users/ Since you have a new group (I saw the posting on r-help, and immediately responded due to it being in HK), you might take a look at how BARUG does things.
At the time I joined BARUG about 3 years ago, we had something like only 20 members. We never dreamed that it would grow to over 2000, which is what we have today! Of course, probably only 10% of those are active, but it is quite amazing, given the relatively small size of the Bay Area, albeit being a tech-heavy region.
We use the Meetup software, which has proved very useful for dealing with the size. (Several other R user groups use it too.) We meet once a month at various venues, ranging from Google and Facebook down to Hurricane Electric (a small ISP). We always have a speaker or two, and various companies have supplied food (Revolution Analytics, O'Reilly).
If any of you visit the Bay Area, please let me know beforehand, and I'll arrange for you to come to our meeting.
-- Norm