I don't know of any but would like some references if anyone does. Of course not all atheists or irreligious people need to belong to an organization.
Studies on the demographics of atheism have concluded that self-identified atheists comprise anywhere from 2% to 8% of the world's population, whereas irreligious individuals represent a further 10% to 20%.
https://en.wikipedia.org/wiki/Demographics_of_atheism
JW LEAKS