The person who joins late may bring a useful perspective the regular members no longer have.

Groups accumulate assumptions the way a room accumulates furniture, and after a while nobody bumps into anything. Someone who joins late has not learned where things are, so their questions come from genuine unfamiliarity. That is the only free audit a settled group is ever offered, and it lasts about a month.

When a new person asks why something is done a particular way, resist the potted history. Answer as though the question were still open, and write down any answer that turns out to be only habit.