fin, actually group interviews are better than one-on-one because there is less focus on you. my suggestions:
sit near the person conducting the interview
sit upright, don't slouch back in your chair
don't be afraid to play off other people's answers, incorporating your own twist, when it gets to be your turn
take interest in others' answers, nod when you agree. don't overdo it, just appear like you are paying attention, even when it is not your turn.
bring a nice business style folder to take notes in. even if you don't end up taking notes, it still makes you look good
don't worry. it's much easier than one-on-one. get there early, too, so you get a good spot!