Leaders count heads. They notice the empty chair before the meeting starts.

Attention to who is missing is a small habit carrying a large signal. It tells everyone present that people are tracked as individuals rather than as attendance, and it tends to catch problems early, since absence is often the first visible sign that something is wrong. Most groups notice a missing person only once a task fails.

Look around before you begin and register who is not there. Send that person one message afterwards, about them rather than about what they missed. It takes a minute, and it is how people learn they would be noticed.