This situation is easy, when you come back and you see a bunch of guys talking to her you cut right into the middle, kiss her on the cheek, turn around and shake hands with all of them introducing yourself and making small talk with all of them, joking around and whatnot. Including them all into the conversation.
Once you come in like that, you already dominated the situation, the guys will either leave or make small talk with you, joke around, and your girlfriend will know she has one hell of a confident man that can handle pressure.
If on the other hand you try to storm in all mad and whatnot, your value will immediately drop as you show you're insecure and can't handle it.
Or, if you just try to leave her alone in the situation and talk to someone else. She may get the impression that you really don't care to be with her or you're just scared.