You get gender balance (or closer to it) by having members invite women they know who seem like they might be interested. I got involved because a friend and fellow-LessWronger invited me. Thinking about the other women... we have a couple who were there from the beginning, a friend of one of the old-timers, Divia whom people knew from the West Coast, and some others whose "origin stories" I don't know.

I don't think you have to paint it pink or anything. Just ordinary networking. (You might possibly have an extra challenge in that San Francisco has more men than New York, proportionally -- but at the scale of an in-person meetup I'm not sure that matters.)