If in a particular society atheists are very less, the few who do not believe in god may stick together in a group.
But the moment the number increases beyond a critical number they tend to split into various interest groups.
Thus we will have atheist groups which primarily stand for social justice like humanist groups, feminist groups, LGBTQ groups etc. There may be politically Left and Right atheist groups. There may be post modernist or regressive Left atheist groups. There may nationalist or sub nationalist atheist groups. There will also be minority phobic, ableist, racist/casteist atheist groups, anti-feminists atheist groups and anti-gay atheist groups.
When atheism becomes a commonly accepted philosophy why one should stay in the same group as another with very intolerable views ?