Two well dressed men at the door.
“Have you heard the Good News about there not being a God?”
I’m kinda sorry I even brought up this subject!
I’m not trying to make atheists out of “religionists” or religionists out of atheists!
I just asked a question, I wasn’t trying to force a view on people!
I guess here (the SDMB), being curious about the “wrong thing” and then having the nerve to actually ask a question about that thing can get a person painted as an ogre really quick!!
Yeah, 'cause the SDMB is so intolerant toward atheists.
What?
I think there’s a degree of confusion here, because Christian missionaries traditionally do two things:
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Explicit, formal evangelism, seeking and instructing religious converts.
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“Works of charity” in the areas of education, healthcare, community development, etc.
I say “two things”, but in fact from the Christian perspective these are merely different facets of the same thing, which is the call to live a life inspired by the gospel. It’s all mission.
Secular observers may assume that, for all missionaries, the “works of charity” are ancillary to the primary motive of seeking converts. This may be sometimes true, but it’s not generally true. Consider, for example, the Medical Missionaries of Mary, a Catholic religious order whose particular apostolate, in the jargon, it is to provide a service of healing in areas of great need. They are clearly and explicitly inspired by religious faith and, as their name suggests, they see themselves (and are generally seen) as missionaries. But they don’t engage in formal evangelisation at all. They are missionaries, unless you arbitrarily adopt a rule that only explicit Christian evangelism is true “mission”.
So are there atheist missionaries? Hell, yes – there are lots of atheists who seek to live lives in conformity with, and inspired by, the ethical values which they hold as a matter of faith, and who seek to change the world in a way which gives effect to those values. As it happens, the faith on which these values rest is not a religious faith – given that they are atheists, and all – but it is a faith which, in many instances, leads them into work analogous to, or even indistinguishable from, instances of Christian mission, working with the poor, the ill, the alienated, the vulnerable to provide healthcare, education, community development, etc.
As it happens, there are also atheists who engage in mission in the evangelical sense, proclaiming their convictions and inviting others to share them, though perhaps comparatively few. But they do exist; Richard Dawkins is an obvious example.
What’s lacking, I think, is an organised movement of atheists who engage in “works of charity” specifically inspired by their atheist beliefs, or for the purpose of promoting their atheist beliefs. But that, I think, is simply because atheist belief is defined negatively – atheism is the lack of any belief in a supernatural god. It’s very hard to reason from that negatively-expressed first principle to a code of ethical conduct; you have to have other principles to do that, and those other principles will be consistent with atheism (as in, they won’t be theistic), but not consequential upon it. So their may be secular missionary organisations through which atheists can and do participate in mission – e.g. Médecins sans Frontières – but there are unlikely to be explicitly atheist missionary organisations, except those focussed purely on evangelism. And I doubt that there are many of those.
It wold seem unlikely for atheists to proselytize, for the reasons already mentioned, but I am reminded of the time some missionary came to my doorstep and mentioned that just a few years ago, he believed in evolution. So I asked him whether he had gone around ringing doorbells to preach it. Lacking irony, he was not amused.
I find it funny that people on the Dope question the existence of atheist proslytizers. Do you guys go to the same message board I do? There’s at least one in every religious debate ever started here. And, in almost every other debate someone tries to convert someone else’s beliefs. Heck, it’s pretty much the point of debates.
With all these proselytizing atheists on the boards I’m surprised they haven’t formed an organization. Then when the OP asks about atheist groups or organizations there would be at least be one. 
Before the humorless respond, I’m perfectly aware that atheist organizations do exist. But they’re mightily thin on the ground. And while I spend an inordinate amount of time reading magazines and newspapers, watching the evening and cable news, and scanning the Internet, these atheist groups hardly ever get as much as a whispered side mention.
The reality, despite the OP’s contention, is that atheists do as little organizing as they do proselytizing. Not proselytizing appears to be an important attribute of the vast majority of atheists. I suggest to the OP that telling atheists they not only should but do act similarly to proselytizing believers might be considered a bit offensive and that you might want to know a skoosh more about a group before suggesting behaviors on their behalf.
Not that atheists constitute a group, of course. 
Bumping this to highlight a new organization, Foundation Beyond Belief, a humanist charitable foundation that provides funding for a variety of secular charities on a quarterly basis. They choose one charity in each of these fields - Peace, Child Welfare, Animal Protection, Education, Health, Environment, Human Rights, Poverty, and “Big Bang”, which focuses on very small charities with small budgets and attempts to provide a “Big Bang” of funding.
I think a lot of posters here might like the idea of this.
P.S. I’m not flacking for this organization, this is in direct response to the OP.