It's always nerve racking to meet a girl's family for the first time. With her friends, it's likely they share her sense of humor or have something else in common with her. So if you get along with her, it's almost guaranteed you'll get along with her friends.
But her parents are a different story. Even if they do share the same sense of humor as you, you'll still find the need to hold some thoughts back. Sex jokes, for example, are off the table unless they're subtle.
A simple rule to help get by is that if you won't say it in front of your parents, don't say it in front of hers. Outside of that, have fun and let the good times roll.