What does it mean when a guy looks at you but ignores when he's with his friends?

It means that he likes u and his friends might be checking the scene out or they like .... [or] if you guys know or have seen each other before it might mean that the guy has a thing for you and has mentioned you to his friends. now that you're there he doesn't want to blow his cover in front of you [no guy wants that] therefore he looks at his friends implying that you are the girl he's been talking about. then the friends look to "check" you out then they share their opinion with the guy once you're gone. P.S. this is usually a good thing