Just be yourself! It's a good opportunity to see if you are likely to enjoy working with these people. Don't try too hard to impress, just be enthusiastic and interested in the proposed project and the work that the other researchers are doing. As for what to drink at the bar (your next question), while it's not a good idea to get plastered, I'm sure you'll be alright to have one alcoholic drink (if you want one!) especially if others are drinking. But if you're worried, have an orange juice. Good luck :o)