It was a long time ago now since i went to that assessment for NFU, but as I can remember it what I had to do was. There were 5 of us in a room, with 3 assessors sitting in the room with us. Usually i would feel really nervous when in that situation but for some reason feel too bad, but maybe that was just me.
The gave us a sheet of paper, with a list of 10 skills that an someone applying for that position should have, then we had to order them ourselves 1 to 10 which was the most important, we had about 5 minutes to do that, then we had to discuss for 20 minutes between a group and reach a concensus on what the order should be.
Obviously you have to voice your opinions on WHY you think a skill should be a certain number. So long as you stick by your choice and give valid reasons why this skill is important you will be fine.
They may change the actual task that you have to do, however it will generally be the same i assume, have a group look at the task for about 5-10 minutes and discuss. So this will im pretty sure be ordering some some different things and then saying why you think its so important.
As the advice goes on this website, (Which is excellent). Best i can advise, is dont be too nervous everyone else is in the same boat. When looking at the task think up arguements for why something is important/not important.
Talk with confidence and clearly put your points across.
Listen to others, and make eye contact with them.
Sometimes give a reply to their ideas eg: Yes I agree because… No I dont agree because… Its imporant you always give clear explanations.
Try not to interupt other people while they are talking. However do not let yourself be dominated eg: dont keep quiet for a long period of time, when someone has stopped talking try and make a point quite often (Not all the time dont be dominating, its about finding a balance be involved but not dominating).
Above all DONT argue, Ive seen this happen before when people get too enthusiastic which can be detrimental to their chances.
In the end I would say you are unlikel to Meet these people again (the other candidates), so if you feel you are out of your comfort zone and are nervous just think Screw it i dont care if i look stupid talking I probably wont see you again
Hope this is helpful in some ways. Though i would recommend you read the useful tips on this site. When it comes to the specifics of the NFU assessment it was nothing too complex, just simple ordering and discussion, the Task is always just there to stimulate discussion so its about your general behaviour and attitude in these situations.