<human>: Can you provide a brief overview of the text?

So I've been best friends with this girl for about 4 years now and we started dating 9 months ago. It's been going great so far, no major problems, but today she told me her new-ish [18M] friend is having a few friends over for a birthday party, and they'd all be spending the night. Now, I've never met this guy OR any of the other guys attending, (she'd be the only girl), and this whole situation is making me extremely uncomfortable. She's reassured me that he hasn't tried anything, and if he did she would immediately leave for my house (I live a couple blocks away from him). I let her know how uncomfortable it made me feel, and her takeaway from it seems to be that I don't trust her and that her reassurances should be enough to put me at ease. So reddit, should I be worried? I don't think it's really my place to expressly forbid her from doing anything, but at this point should that be my next course of action?
<bot>: My girlfriend wants to go to her male friend's birthday and sleep over, I am uncomfortable with this and have talked to her about it. What's my next course of action?