Sometimes the group didn’t change. You just finally stopped laughing at the joke you never liked.

Sometimes nothing happened to the group at all. You had been laughing at something you never actually found funny, out of wanting to be inside the circle, and the change is that you stopped performing it rather than that anybody drifted. That is honesty arriving, not friendship ending.

So start small: do not laugh, do not explain, just let the moment pass. Say the mild version of what you think when the joke is about somebody. Some of them will follow you, and the ones who do are the real friendship.