Yes I attended an assessment and no, there is no preparation you can do. You will have 3 group tasks to complete, your group will keep changing so you don't get too friendly with the group. The trick is to be yourself. Nobody knows exactly what they are looking for so it's a waste of time trying too hard. I am not vocal at all, and to be honest hardly said a word although, what I did say was relative and valid. Don't let the mouthy ones sway your decisions. Go with what you feel and make valid, backed up points. I can't say whether I will be successful or not, they really give nothing away. Just be yourself and relax. It's in the hands of the assessors.