1. I have recently spoken to some consultants at Monitor Deloitte (not in the Middle East) about the group case, there is really not much you can do to prepare aside from practicing case studies. You must be able to work well with others, do not feel pressured to lead the group unless you are very comfortable doing so.
2. Do not go into the interview expecting it to be candidate or interviewer led based on what you've read online about that particular company/office (I learnt this the hard way), prepare for both types of interviews because it really boils down to the interviewer's preference; some will let you take the wheel and drive the case whereas others will approach the interview differently. During my final round at BCG (Dubai), my first interview was very interviewer led and the second was candidate led.