Be polite, shake their hands firmly, look them in the eyes, remember their names and use them in conversation, get the hell away from them as soon as possible, and do your best to make sure they don't come along with you and your girl.
Basically, you don't want to seem intimidated by them. You want to be friendly and open without giving away too much information about yourself, and you don't want their pre-existing relationships with the girl you are interested in disrupting your game.
Depending upon your "arrangement", you can use the "we are going to be late" excuse to ditch them.
Also, I don't think it is necessarily a mistake to meet her friends, although the circumstances here are not ideal (namely all of them and one of you). A chick's friends influence her a lot and making a decent impression cements your position. Besides, you have gone out with this girl for a month so you are going to have to meet her friends eventually. Just know that they are going to gossip about you when you leave.
Oh yeah, and you can check them out to see if any of them are hot or not. ; )
Peace.