A favorite Internet chestnut is that atheism is a religion like not collecting stamps is a hobby (alternate version: like bald is a hair color). I'm sure whoever came up with that is proud of himself (since vocal atheists are almost exclusively male, I feel pretty safe saying "himself" here).
Fair enough. It's a catchy statement. So let's check in on the latest meeting of the local Non-Stamp-Collector's group, shall we?
Reg (the group leader): Let's all welcome Judith to the Non-Stamp-Collector's group.
Group: Hi, Judith!
Reg: So, Judith, did you ever collect stamps?
Judith: Yeah, when I was a kid. My parents are still avid stamp collectors.
Stan: Hey, Reg, have you had a chance to read Why Stamp Collectors Have It All Wrong yet?
Reg: Not yet. Did you crash that Internet poll: "Is stamp collecting totally stupid?"
Stan: Oh, yeah! We totally turned that around. I don't know why stamp collectors can't see the fallacies in their arguments.
Judith: (wonders if they're ever going to talk about something else)
OK, that's enough of that (yes, the names are shamelessly pilfered from Life of Brian). Do you see where I'm going with this? Judith knows she's not into collecting stamps, despite it being the predominant hobby. But she was kind of hoping for suggestions on alternative hobbies, and she isn't getting any. And if you spend most of your time complaining about stamp collectors, well, then that is a hobby.
At this point I think the metaphor is obvious enough, so I'll dispense with the references to stamp collecting. I actually don't want to condemn these types of groups outright, because they certainly have value. It just depends on what kind of group you want to have. For some people it's important to have a safe space to air their grievances. But if you're going to be that kind of group, your membership will almost certainly remain small.
For people who want to expand their freethought groups, it might be necessary to lay off the bashing of those who aren't hardline atheists and incorporate activities other than talking about what they don't believe and playing "spot the fallacy." I brought a friend with me to one of the groups I hang out with, and she's been afraid to come back because she felt like people were looking down on her for identifying as agnostic instead of atheist.
It might sound like I'm saying the second kind of group is better than the first, and I'm actually not. Like I said, safe havens are important to have. Just realize it's going to narrow your group's focus considerably. If you're ok with that, keep on doing what you do. Just don't wonder why more people don't come.
For even better phrasings of this argument, check out this post on The Meming of Life by Dale McGowan. He's a professional. Trust him.