When the joke is about the new kid, laughing along is a vote.

Laughter is not neutral, it is a count. Whoever is telling the joke is reading the room to find out whether this is allowed here, and every laugh comes back as yes. You can mean nothing by it and still be counted, because the person being laughed at is counting too and cannot see your intentions.

Not laughing is enough, and turning to the new kid to ask them something ordinary is better. One person declining to join is often all it takes for a joke to stop being worth telling.