The person left out may notice the boundary before the people inside realize they made one.

The person outside a boundary notices it long before the people inside do. Those included experience only warmth and familiarity; the excluded one experiences the edge of it, sharply, and is often the only one in the room aware that an edge exists. Belonging is invisible from within.

So when you are inside a group, do not rely on your own comfort as evidence that everyone is comfortable. Watch for who is quiet, and address them by name into the conversation rather than waiting for them to find a way in. It is a small habit and it is close to the whole of making someone welcome.