There are plenty of secular groups that have cohesion (either between entire family units or between people without the rest of their family): non-religious sco

There are plenty of secular groups that have cohesion (either between entire family units or between people without the rest of their family): non-religious scouting organizations, hiking groups, various volunteer groups, some political groups, gaming "clans", fan clubs, neighborhood groups... All of them are easy to join and leave and have churn, just like churches do when people move in or out of their region.