What does it mean when a girl has a really close guy friend?

I've been dating this girl for about a week now. We haven't made any official commitments. I believe that she's into me because of the fact that we have become really close. Now, I went out to this concert with a group of her friends (as well as a really good guy friend of her's).

At this concert, for the first bit, she was dancing on me and everything was fine, but then she asked to dance with her guy friend. After dancing with him for a couple songs, I started feeling left out. After they were done, she danced with me the rest of the night.

However I still feel slightly uneasy about the guy friend. Am I just being foolish for feeling uncomfortable or should I express some concern to her?