Somebody always ends up off the edge of the group photo. Notice them.

Groups have an outside, and it is usually occupied by someone who did not choose it. The edge is not always obvious; sometimes it is the person who laughs half a beat late, or who never quite gets asked. Most exclusion is momentum rather than cruelty, because nobody feels responsible for a gap they did not create. Noticing is the thing that breaks the momentum.

At the next gathering, find whoever is standing slightly apart and go stand there too for a few minutes. Ask something ordinary. You are not rescuing anyone, just moving the edge a little further out.