The group changes the norm
Inside a group, behaviour repeated by many people quickly begins to look normal. Language, humour, acceptable risk, attitudes toward outsiders and even definitions of success can shift without any formal order.
This is one reason groups are so effective: they reduce the cost of coordination. The same property also allows a mistaken norm to travel from person to person.
A dual position
An observer does not stand outside with contempt. The person participates, performs a role and uses group resources while simultaneously tracking how their own criteria change.
Useful questions are simple: what did I begin to call normal only after joining this group? Which claims are not usually questioned here? Which behaviour is rewarded? What happens if I disagree calmly?
Exit criteria before emotional dependence
The best time to define exit conditions is before status, friendship, money or identity becomes dependent on membership. Later, every evaluation becomes mixed with the real cost of losing the group.
This does not mean leaving groups frequently. Clear criteria can make long participation more stable when the system still serves its purpose, and make departure more honest when fundamental conditions change.