that sounds a bit weird to be honest. how are things going with you and you're girlfriend?
some girls like hanging around with boys, doesn't mean you shouldnt trust them though. I've had girlfriends that have hung around with other blokes one on one, and it never bothered me. Its only when you notice the signs that she's treating you differently, that you should bring it up. And if it still bothers you, and things dont change, you have to bite the bullet sadly.
The worst thing you can do, is throw a tantrum about it, which i have learned to my expense previously, as it makes you look insecure and jealous. Not the confident male you should be.
On the other hand, the thing you should be doing is then spending time with your own friends as right now you may be too available to her. Do it back, be busy every night one week either by friends or work, and she'll realise what she's missing and come crawling back. They always come crawling back.
Be honest with yourself if things start to change for the bad. But remain aloof at all times, do not act up about it too much. If it really bothers you, and you tell her, but things dont change then jealousy will manifest itself and then things will go downhill. Resulting in eventually her losing interest as she'll see it as you being clingy. Which is why (if you need to break up over it) you can use her hanging around with too many men all the time as one of your reasons. That way, you havent got upset or thrown a tantrum, but you've stuck to your principles and she hasnt cut it.
That way, you come out looking the big swinging **** you are, as you dont let no woman put you on the side.