On finding this picture, it just occurred to me that a lesson in manners might be all it would take to eliminate anyone from feeling left out. A simple, "Hey, Sally!" as a recognition would give the girl a sense of value. Inviting her into the conversation or group would increase that value exponentially. Sally can graciously choose whether or not to join the group, but at least she would have the esteem to make that choice with no hard feelings.
Something as simple and overlooked as manners can go a long way in the development of a child's self-esteem and character. Let us not forget the addage, "Do unto others as you would have them do unto you."