Without reading any other replies (which I will do after posting mine, probably much to my dismay
) I can understand why some atheists would be reluctant to join an organization based on atheism.
One of the fundamental beliefs of many atheists is that organized religion is problematic. When you get a bunch of people together who are all preaching the same unique ideas, they tend to stand by those ideas due to a sense of loyalty to the group. They gain a sense of belonging from it, and it becomes a faction. This sort of congregation can be dangerous. But congregation is one of the tenets of our society. (USA) So religion is tolerated in all its forms. This is also why the KKK continues to exist. When you have the right to congregate, you take the bad with the good.
Also, many atheists (myself included) have reservations about joining a "club." Our beliefs are our own and we don't need others to validate them for us. By joining a group, you lose your individuality and contribute to an amalgam of ideas; some of which may contradict your own. That leads back to my first point. If you feel inclined to be a part of the group, you will protect its ideals. Thus, you gain a sense of belonging and become a faction.
The only way to be part of a group based on beliefs, and maintain your individuality, is to stand opposed to the group. Also known as: traitor. Expect that title if you follow this path.
Believe me, I've considered trying to congregate the local atheists. But the only way to do it safely is to make it casual and require nothing from anyone. Ever. That's counter-productive, but it's the only safe way to unite them.