It's said that organizing atheists is like herding cats. We're implicitly compared to believers, who have acknowledged leaders, authoritative texts, and formal organizations. The reason usually advanced to account for this phenomenon is that we atheists are generally rather individualistic and thus reluctant to follow someone else's lead on such matters. But there's another reason which I've never seen presented in the context of explaining the herding-cats idea: atheism is much too broad a concept under which to seek to organize. The proper comparison is not to individual religious sects but to theists as a whole.
If we randomly put a dozen people who don't believe in any gods in a room, then they're no less likely to reach an agreement on any given issue than if we randomly put a dozen people who do believe in gods in a room. A representative sampling of a dozen of the world's theists would include four Christians (two Catholics, one Orthodox, one Protestant), three Muslims (two Sunnis, one Shiite), two Hindus, a (theistic) Buddhist, a Taoist, and an Animist. I rather doubt they would agree to much at all, since they don't even agree on the basic definition and identity of the gods. In fact, the random group of atheists might even be more likely to reach agreements since many atheists have great respect for science, which provides an objective way to establish underlying facts, since appeals to faith would be roundly rejected, and since atheists have no prejudices against other atheists who don't believe differently than they do. The task of organizing atheists seems significantly different from this perspective.
I'm not expressing an opinion about whether atheists should attempt to organize. Atheists—as distinguished from the much larger group of the non-religious—number very few in many parts of the world, including my own, so there may be wisdom in trying to gather as many people under the umbrella as reasonably possible. My point is only that organization works best when it's structured around a specific set of beliefs and not a general belief or disbelief. Only with that in mind can one make real comparisons between groups. Link