Inside jokes are fun until they become a wall. If the new kid can’t laugh along, explain it or drop it.

Shared history is one of the nicest things about a group and also a fence with no gate in it. Nobody intends that: you are just talking normally, and the newcomer sits there doing arithmetic about how long until they get the references. Every repetition quietly confirms that there is an inside and they are not in it.

Spend ten seconds explaining the joke when it comes up, which costs nothing and includes them immediately. Ask them something only they can answer. One deliberate hour of that and they stop being the new one.