Inside jokes are how a team writes its own history. Make sure the new guy gets let in on them.

Shared jokes are a group’s private language, and a language is also a border. Every reference is a compressed memory of something you all survived together, which is why they feel so good from the inside and so alienating from the outside. A new person hearing six of them in a row learns that there is a room they are not in. Nobody intends that, and it happens anyway.

Do the two-second translation out loud. Explain the joke to the newest person even though explaining kills it a little. Better still, build one with them in their first week, so they have something of their own in the pile.