One suggestion that I would make is to be careful to be inclusive, and not allow the focus of the group to wander.
The atheist group that I've attended has been taken over by radical political liberals, and has gotten essentially turned into a liberal club, with a political litmus test required for membership. Anyone who disagrees with their extreme political agenda is unwelcome; atheist or no.
So my advice is to emphasize your commonalities, and discourage cliques based on differences. Things like that can kill a group quickly.
I am a member of a newly formed Humanist group. We have about 30 members and are a member of the American Humanist Association. I would say about half of our somewhat small group are members of my UU church. Most of our meetings are at my UU church. We are just starting out like your group and we are still sort of defining our purpose and goals. But being a member of a national groups helps. Did you consider joining the American Humanist Association or a similar secular group?
I should mention that the guy who started this UU group on Atheist Nexus is the head of our Humanist Group, South Jersey Humanist.
My wife and I started going to a UU church over 5 years ago. At that time she was looking for a Pagan group and I was a wavering agnostic/deist making a last ditch effort to find some religion that I could have some confidence in. Today she's agnostic and I'm a full blown atheist. Our minister turned me on to Bertrand Russel when I approached him about my non-belief. A friend started a "Humanist Discussion Group" at the church Those who attend the group aren't all on the same political page, nor are they all non-believers. The discussions can get heated, especially when we hit some political hot-topics or discussions on objective morality, but everyone goes home friends. It's fantastic group.
Anecdote: I was in Boston a few years ago and stopped by 25 Beacon St. to visit the UU headquarters. I bought my first book on Atheism in their bookstore; The End of Faith by Sam Harris.
I hope your group is going well. There certainly are others. Did you find the UU Humanist group?