One thing that has not been mentioned yet and is very relevant to group exercises... group exercises always have a TIME limit on them so it is always a good idea right at the beginning of the task to volunteer to be the time-keeper and make sure you let your team know towards the end how many minutes you have left ! There's no point to volunteer to be the time-keeper and then forget to check your watch ! Also, always watch your posture: slouching is a big no-no and guys, keep your legs closed ! Try not to fiddle with your hands, if you have a pen in your hand, PUT IT DOWN 'cos you'll automatically start playing with that if you're nervous and there's nothing more distracting to the interviewers...
One other thing as far as grooming is concerned, POLISH YOUR SHOES !!!!!! And Ladies, bring a spare pair of tights with you just in case you ladder the ones you are wearing. Flesh-coloured tights are better than coloured ones by the way.
And remember at the end of the interview to thank the interviewer and say goodbye with a firm handshake...