Isn't it weird that people sing love songs to a killer-spirit on some day of the week that that killer-spirit reputedly says is special? Yes, if that singing is seen to be conventional, then we don't see it as strange because even people who don't do it have become desensitized to it. If people were to gather in some place to chant to what they thought was a being from another planet, then most of us including the religious would think they were off their rockers. Chanting to a being from another planet isn't conventional, so we wouldn't accept it as sensible. So whether we see the act as odd or not depends on what we're used to.There's conventional paranormalism and unconventional paranormalism, maybe that's the best way to say it. Woo is woo whether it's popular and normalized or whether it isn't. Going into a magic building on a Sunday to sing songs to spirits is considered pretty rational in some circles. If you see it as group meditation then maybe it is.
Here's another illuminating illustration: Say I tell people I worship an evil spirit that tortures and mass-murders people and sends young girls into sex slavery. If I tell people the spirit is Satan, then most Christians would angrily denounce me as a Devil-worshipper. If I tell them the spirit is God, then they would admire my spirituality.
Anybody can fall into the trap of groupthink including atheists. Any atheist groups that encourage groupthink are no better than Christian sects, in my opinion. Most atheists, I think, are not members of such groups and may feel no reason to join them. That freedom to be independent I see as the advantage of atheism.In the end it's the behavior that matters. The insidious element in all this is how far a person will go to preserve their group identity, whether they accord the same acceptance, respect and rights to non group members or whether they allow harm to occur to others because they are not perceived members of their preferred tribe.