Now comes the balancing act. Your girlfriend may want you to take an interest in her friends but don't go overboard. Getting to know her friends is one thing but if she senses you're flirting (and women are very in tune to that sort of thing) or ignoring her in your effort to be friendly, it can cause problems or doom the relationship. Exercise caution when meeting her friends. Take an interest but not too much.
For a healthy relationship don't go overboard with her friends, especially if they seem infantile.
Listen carefully when you meet her friends for the first time. Try to hang on to as much information about them as possible. After the meeting when you're alone with your girlfriend, make a casual comment or ask a question RELATED to the earlier meeting. This lets her know that you were listening and that you have a real interest in getting to know her friends. Again this is a balancing act but for the most part you can pretty much figure that saying "you're girlfriend with the blue dress and cute dimples looked hot" is asking for trouble.
Your girlfriend's buddies are probably going to play a significant role in whether or not the relationship survives. Yes, it can seem like one long job interview but try to make a genuine effort to get to know her friends. Since you will probably be spending quite a bit of time with them, starting a courteous, respectful relationship can only make your girlfriend happy and your life a whole lot easier.