Get her to fit a lock on her bedroom door, maybe? Other than that, all you can do is express your dissatisfaction and trust her judgement. If you trust her, and your relationship is sound, then there's no problem.
For example, my partner has often been to see his son, who lives with his ex (the mother, obviously), and because they live so far away, he has to sleep over, in her living room. I have no problem with this, as he needs to see his son, and his relationship with his son's mother was over and dead years ago, they're just friends now. She also has a new partner, so there's even more reason to trust. I also fully trust my partner's judgement, having been with him for nearly ten years now.
But if there's any doubt about your girlfriend's judgement, maybe the relationship isn't as sound as you'd like? You really have to be open and honest about how you feel about this situation with her, and see what she says. And of course, fit a lock on her door.
Meet her flatmates too, and see if you like or trust them, get to know them, maybe make sure to spend a couple or more nights a week over at hers? If you all become friends, then they're probably more likely to see your lass as "off-limits".
Good luck.
