I understand what you're going through..
How long have you been dating your girlfriend? If you've been dating a short amount, chances are you don't trust her that much, in which case it's natural to worry about her cheating on you (just try to limit the accusations). If you've been dating her for a while, why are you worried? You two already put so much time and effort into this relationship, I doubt either you or her would screw it up now..
It's natural to feel the way that you do - you don't want to get hurt. Just ask her what she does - don't spy behind her back or accuse her without proof! Ask if next time you can tag along. If not, go hang out with the guys.
Girls are just like guys when they're hanging out.. they just sit around talking while doing some activity.. (and generally, girls see shame in cheating). You have no reason to worry.
Remeber, she has the same right to accuse you of cheating, but you know that you're not. Put yourself in her shoes, I guess, is what I'm getting at.
Relax a little.. girls come and go! If it doesn't work out, it doesn't work out, don't upset the relationship now, which could potentially end it. Relax and ride things out!
Hope I helped a little..