Sometimes you will get girlfriends who will flirt with a friend of yours like this. If she likes you, odds are she will like your friends as well... birds of a feather flocking together, and such.
Simply not bringing her around your friends often is one solution to it. I think that's a fine solution if she can't behave herself.
The ideal solution though if you want to get her to knock it off without making yourself look jealous or weak is having her bring some of her friends out when you bring yours, and if she starts flirting with your buddy, you start hitting on her friend. When she confronts you on it later (she will) the conversation goes like this:
HER: It seemed like you were getting pretty close with Mina.
YOU: We were just talking. She's a nice girl.
HER: I didn't see you talk that way with anyone else there.
YOU: What do you mean? I mean, normally I wouldn't, but I've seen the way you flirt with Joe, so I figured you were okay with it.
HER: Huh? Joe and me? We're just talking! There's nothing there! I don't even like him!
YOU: I know, exactly! That's just like me and Mina! Did I do anything with Mina you did not do with Joe? Of course not!
HER: Yes, but with Joe you know nothing will happen there. With you and Mina...
YOU: Don't be silly. I only talked to Mina to give you and Joe some space. You seemed pretty excited to be talking to him and I was like, "Well, I'd better go do something else!"
HER: Mmm... well... hmm... I don't know...
After that you get a period of introspection, after which you will notice that a.) suddenly she is on her best behavior around Joe, and b.) you and Mina will almost never end up in the same place ever again, lol.
IMO this works a whole lot better than direct confrontation, because if you go and start talking to her it makes you sound needy and jealous.
If on the other hand your response to her flirting is "(shrug) Guess I'll go flirt too", she will definitely notice that and definitely get jealous.
Most girls are not fully consciously aware of what they are doing. So, again, when you just point it out, it makes you look petty for noticing something she was barely paying attention to. However, when she is the one to bring it up, because you triggered jealousy in her by hitting on one of her friends, then you get to do the whole "Whatdya mean? I'm just following your lead. I thought it was okay since you were doing it!" bit.
Then every time she tries to make it about the girl you were flirting with, you bring it back to how she was flirting with that guy first, so you figured it was fine.
At which point, the introspection starts... and then she concludes, "Ohhh. If I don't want him hitting on my friends, I'd better not flirt with his."
(there is also the secondary realization that, "Oh gee, I am really not behaving like a monogamous girlfriend around him. He is acting like I was his FWB or something there and saying it was just a response to my behavior. Yikes, I am making myself look like a bit of a skank... I didn't even realize, but he and other people definitely did. Crap")
Note that this will not work with female players... who will not care if you are hitting on their friends, and will assume you aren't bothered by them hitting on yours... nor with girls who are super pissed off at you or ready to ditch the relationship, who know they can always escalate faster with men than you can with women, so if both parties are going no-holds barred it's a game you cannot win.
Within the confines of a normal, otherwise healthy relationship, where a girl is probably not fully conscious of what she is doing, this is the best strategy for resolving it IMO.
Doesn't make you look weak or petty like a confrontation does; gets her to be the one feeling jealous and doing the chasing; and still causes her to reflect on her behavior, totally on her own, without you even asking her to, due to the magic of
operant conditioning.
Chase