The spare chair matters most when someone arrives without knowing whether there is room.

An empty seat says something before anybody speaks. Arriving at a full table means calculating whether you were expected, whether to make people shuffle, whether your presence is a cost, and most people would rather leave than ask. Keeping room is a form of welcome that works without anyone having to be brave.

Set out one more place than you need. Leave the gap in the circle. When someone hesitates at the edge of a group, say the sentence that removes the question rather than waiting for them to ask it.