Your girl's friend obviously thinks you're a drag on their friendship. The best way to approach friends like this is to have fun.
What your girl's friend (and all the others most likely) don't want is your girl to have a boyfriend who just rains on everyones parade. For example a guy who goes out with his girl and has to keep her all to himself. Not only is this the sign of insecurity, but it also shows the lady friends you're possessive and too tight up for any girl. You should be practicing not being like this anyways (not saying that you are).
I find that when you give your girl's friends attention as well then they sit back and stop with the jealousy insecurities. When I say attention, I don't mean forced attention. I mean playful joking fun, maybe even a little flirting.
You say flirting? Yes I do. I used to flirt with all my girl's friends. Not only does it show that you are a worthy male for her friend (your girl), but it also shows that you are willing to let go and have fun around her. You're not a possessive creature. This frees the chains that your girl's friends may have on her when it comes to you.
Here and there you may get some flak for speaking openly to your girl's friends, but I'd much rather have jealousy coming from my girl than her friends. These are the friends that can prove fatal to a relationship. If they do not like you or enjoy your company, they will relentlessly try to detach her from you. They will take her out clubbing in hopes of her finding a new male. However if you show them that your an ample man who can show them fun as well, they will instead love the fact that you're around. I've been on both sides of the quarter and I can tell you being down with the friends is the way to go. No matter how ignorant or annoying they can be. All it takes is a few minutes of your time.
