Come introduce yourself, play a game, and have some open conversation to help improve your use and understanding of German. If you're a native speaker, it's a great way to help others and network.
You can be at any level of study, but at least a basic vocabulary and sense of grammar helps. You may otherwise want to bring: a notebook, a writing implement, and perhaps a German dictionary, book, magazine, or newspaper. If you want to prepare, try to be able to answer in German at least some basic facts about yourself, like where you live, what you do for work, and anything interesting that happened to you recently.