When around a group of people, and they suddenly start laughing at a joke you don't find funny, how do you act?

You have pretty much covered it all. It doesn't matter at all when I know the people and feel comfortable with them, in other words, I can be myself. In fact, they laugh that I don't laugh. If I am in a formal situation, I can feel awkward, but I try not to worry about it too much it as I get older.