I suppose that depends on what you classify as an explicitly atheist organization.
Does attending parties where pot is smoked, illicit drugs are taken, and passed around, alcohol served, rock music is played loudly, either on a stereo system, or a live rock band plays, and sexual dalliances take place in obscurity, count?
Such activities took place in the culture I came out of in the 70's.
I'm told such activities have been a significant part of most teenage lives, in many generations, from at least the 30's through to today.
Granted, it probably dates back millennia. Especially in larger metropolises.
The 1920's were called the roaring 20's for a reason.