Why does Atticus not bring a chair for the stranger in the room? Who might this stranger be?

Chapter 28 of To Kill a Mockingbird is quite an exciting chapter. It is in this chapter that Scout and Jem are attacked in the darkness by Bob Ewell on their way home from the schoolhouse. A man intervenes on their behalf and carries Jem home. Soon after, Dr. Reynolds and Heck Tate arrive at the Finch home. Suggesting that everybody take a seat, Atticus proceeds to get another chair from the living room. All of the adults in the room are seated with the exception of one.
Scout believes that Atticus knows "the ways of country people," so it does not strike her as particularly odd that a man is left standing. She assumes that the man prefers to stay where he is. Eventually, Scout tells her story of what happened. As she identifies the man standing as the one that must have intervened in the scuffle, she looks up at his face and says, "Hey, Boo." The man without a chair is Boo Radley, their reclusive neighbor. Atticus knows that Boo would prefer to go unnoticed, so he does not offer him a chair. After a while, the men move out onto the front porch to talk. Picking up on her father's clues, Scout leads Boo to a chair on the porch that is in "deep shadow."