A small courtesy can make a room feel less divided between insiders and newcomers.

Rooms sort themselves quickly. People who know each other fall into shorthand, and anyone without the history stands slightly outside it, aware of being outside and unwilling to say so. Nothing unkind has happened, which is precisely why nobody corrects it. One introduction, one explanation of a running joke, one question aimed at the quiet person changes the geometry of an entire evening.

At the next gathering, appoint yourself to that job. Find the person who arrived alone and give them one thread they can hold, then stay long enough for the conversation to take without you.