The person who is new should not have to become entertaining before being included.

Groups often set an unspoken entry fee: be funny, have a story, keep up with the pace of the joking. Regulars pay it without noticing because they have years of shared material, while the newcomer has none and must audition instead. Plenty of people who would have belonged easily get filtered out by this, and it looks to everyone as though they simply did not fit.

Give a new person a way in that needs no wit: ask about something they know, or bring them into a practical task. Explain the running joke instead of leaving them outside it. Belonging first, entertainment later if at all.