Since I'm a girl, I think I'll give her the benefit of the doubt. Maybe she just likes to hang out with guys and she is ignorant of the intentions they may have. Can't say I haven't been there. The big thing is: Trust is huge in a relationship, and if you have a problem with what's going on, you guys should talk about it....something obviously has to change.
Her intentions may be pure, but if it's going far enough for you to question your trust in her, then one of you guys are in the wrong. Either jelousy on your part, a flirty personality on hers, or something....