Y'all is never correctly used in reference to only one person. It usually refers to a person and his group, family, collegues, or to everyone that is present. The people included is recognized by the context of the conversation or situation. If more than one group is present then "all y'all" may be appropriate. If a true Southerner were to say, "Glenn, y'all come over for some possum." It means Glenn and his family or group.

Jerry Liles