Stand up for people who aren’t in the room. Word gets around about who does that.

Saying the fair thing about somebody who is absent is one of the few reputational moves that travels by itself. Nobody in the group chat thanks you at the time, and it reaches the person eventually, because these things always do. It also changes the room on the spot, since a pile-on needs everybody to keep going and one flat comment can stop it. What people take away is that you are not safe to gossip near.

No speech is required. Something plain does it: that is not really true, or I like them. Then change the subject.