What is the difference believing that there is no God and believing there is one? In my philosophy it's the same concept.
I don't personally know how many members a "group" needs to qualify as a "religion" but I think atheists have enough to meet some sort of requirement or criteria in my opinion.
And like I said - the lack of belief is the same as belief - we both believe in something and IMO, "believe" is what defines a religion.
Obviously this is all really philosophical, and definitely subjective so we can leave it at that.