Guy I'm seeing is "best friends" with a girl who is known to sleep around?

[=)]

Well, I've been dating this guy for the past month. Everything is going by great; we're pretty much a couple, though not "official". I can tell he likes me, and I really like him.
So there's this girl who he talks to. First, she followed me on Twitter.. we had mutual followers so I didn't mind. Then, she added me on Facebook (which was before the guy even did). One week later, I finally met her, and apparently she considers him her best friend.
Then a few weeks back, my friend comes into my room, warning me about her. She said she realizes that on Twitter she mentions him a lot, and on instagrm she posted this picture with him, saying "My bestfriend <3"... again, I didn't care really since I trust him.
SAME NIGHT. He had a movie thing in his room, and I had went. Once the movie had begun, the girl was talking. The guy playfully told her to "shut up". So the girl, with her booty shorts and all (lol), gets off of the bed, and tackles him on the bed, legs sprawled around him and everything. He was laughing and playing along. I was really annoyed, but I didn't say anything, though I did end up leaving early.
Ever since, he still has those movie night things, but he always invites me last minute (it's usually a group of his friends), though for some reason she's always there (though, according to my friend, she invites herself places a lot). I haven't gone since the last time.
Then today, I was talking to my friends about it, and it comes to find out, this girl is known to sleep around with guys randomly and even homewreck relationships. I go to a big university, but somehow every freshmen knows everyone lol, so she told me a couple of stories, but at the same time she told me not to worry about her because "she's not that pretty anyways".

I don't know, now I feel some type of way about this. Some people say I should talk to him about it - but about what? We're not boyfriend and girlfriend. It's just bothering me though... Advice?

[wolfgang]

You're in a difficult situation here and atm as you're not official you have 2 options as I see it: 1) you ask him out - once you're together you can hopefully get him to cut down on seeing her, and he should be able to be committed or 2) you just play it cool, which he will find attractive as you're not clingy (once he compares the two of you, you should come out a clear winner) and in time he will get bored of her. Good luck

[Ava]

wow, i really hate girls like her. sounds like she doesnt really want to share him with you, trying to stake her claims on him by posting that photo and flirting with him and claiming they are best friends and everything... and it really complicates things since you arent actually his girlfriend.. but if he's bestfriends with a girl like that then maybe you should think about avoiding that timebomb by forgetting about him altogether while you still can... because even if you start dating, "talking" to him about her isnt going to make her go away. shes always going to be a threat. unless hes going to get her to stay away, which i doubt.. its really up to you what you do though. are you willing to fight for him? would you be able to handle being with him if he wont give her up? girls like that never really go away unless the guy MAKES them. and its rare that happens.. and when it does, everyone will assume you're the bad guy. ugh.. it just gets ugly. if it were me, i would just stay away from him to avoid that whole mess. if you are important to him, he should ask you out.. and tell her to lay off next time she tries to flirt.. but in my experience, girls like that can never be trusted and guys usually fail to have the proper boundaries regarding them.
but yeah i guess if you really want this guy, you should ask him out. and talk to him about her, and hope for the best. even if you arent dating it probably wouldnt hurt to tell him how she makes you feel, since apparently you two already kind of have a thing going, right? if hes a good guy he will stop flirting with her and put your feelings first, and even tell her to leave him alone if she doesn't back off on her own.
its just, if you couldnt tell, im sort of bitter about this sort of situation from my own experiences.. but I really hope it works out well for you, whatever happens. communication will be needed, and boundaries. good luck.