Seeing is Believing

Who are these people that the new governess sees that come and go? The new governess sees other people at the house and questions who they are. She describes the people in detail to Ms. Grose and finds out that these people have died. This gives me the impression that the people the governess sees are ghosts. Why is the governess the only one who can see the ghosts? I think that Ms. Grose, Flora and Miles can see the ghosts, but they just aren't telling the governess. Obviously, Ms. Grose knew exactly who the governess was describing. This makes me wonder if Ms. Grose knows more than she is expressing.