A shared joke can mark belonging while quietly telling someone else where they stand.

An inside joke does two things at once, and only one of them is friendly. It confirms a shared history for the people who have it and marks its absence for anyone who does not, without a word of exclusion being spoken. That is why a room can feel warm and unwelcoming at the same time, depending on where you are sitting.

When someone new is present, either explain the reference briefly or leave it for later. It takes one sentence, and it decides whether the newcomer spends the evening inside the conversation or beside it.