What you laugh along with, people assume you agree with.

Laughter reads as agreement whether you meant it that way or not. Join in and the person telling the joke is confirmed, the person it was about is counted as fair game, and the room records you as part of it. Most of this happens without any decision, out of awkwardness or timing. The effect is the same as if you had agreed out loud.

Notice the half-second where you are about to laugh at something you do not actually find funny, and simply do not. The absence of laughter is louder than you would expect. If it was about somebody, say something kind to them later, separately.