A group becomes smaller inside when everyone learns which honest reactions are inconvenient.

Groups shrink from the inside without anybody leaving. Once people work out which reactions cause trouble, a look, a sigh, a swift change of subject, they stop offering them, and what remains is a narrower band of acceptable response that feels like harmony and is really absence. Nothing gets announced and everybody adapts. The group keeps its members and loses their range.

Notice what nobody says in a room you are part of, and who stopped saying it. If you have any authority there, respond well the next time something inconvenient is voiced, because that one response teaches more than any invitation to be open.