Inside jokes are the receipts of a real friendship. Nobody else gets them, and that’s the point.

A joke only two of you understand is a small record of time spent together, which is exactly why it cannot be manufactured. It exists because you were both there for something ordinary and stupid. Nobody can join it later, and that is not meanness, it is just evidence. Friendships get built out of shared boredom far more than out of big events.

If a group you are in has no running nonsense yet, it probably just needs more hours. Go do something unremarkable together and let the joke happen by accident. Around new people, keep the old ones short so nobody is left standing outside them.