Why do atheists need a group? We don't have anything in common other than not believing in god. I don't join groups with other people who don't believe in unicorns, not believing in something isn't a reason to join a group.
I hear this a lot from secular, non-religious, and atheist people. I even hear it from religious people. Atheists have a tendency to reject 'groups'. We are lone wolves, who reject society's norms and tread our own path, rebels and freethinkers who break the chains of ideology to free our minds. Or at least that's how many of us like to see ourselves.
Many who break away from religion reject everything associated with it. Religion can cause a lot of harm. Many denominations have ways of suppressing individuality, discouraging free thought and questions, and indoctrinating its members into its ideology. The methods used by churches to accomplish these goals can be powerful and damaging. The church has had a long history of using, not just emotional manipulation, like shame and guilt, but also more direct methods such as excommunication, shunning, torture, threats of hell, promises of heaven etc., to keep people in the church even if they don't believe.
The threat of having your support system, your community, and your family taken away if you don't think like everyone else prevents people not only exploring other ideas, but also from expressing themselves in an authentic way. It can be crushing to feel different from everyone around you when you are part of a church with toxic faith. You love your family, you love your friends, and you don't want to lose them, but everything they believe in is in opposition to what you think or who you are, and it can be a heavy weight to carry.