I see, well, sometimes what this can mean is that this girl fears your daughter and is just connecting to one of the other girls because she is shy. IF this girl tried to pull the others away from your daughter though, she is most likely doing so out of jealousy. And sometimes what can happen is focusing on what girl responds to her, hoping she can eventually pull the others towards her as well. Everyone has a right to have a closer friend, it just happens sometimes. It's when it's something that is pulling that one individual away from others that becomes a problem. It's called singling out, and it's not innocent, it's claiming instead.