Buddhism would be a religion without a god. Atheism is simply not believing in a god.

While atheists may be Unitarians or even form their own congregations (not to mention many Atheist Buddhists), this are segments of the community, not atheists as a whole. Some find a certain appeal to the social benefits provided in congregational life and we certainly have plenty to discuss at group. But to imply that atheists as a whole participate in such a life is like saying all Californians enjoy surfing.