Congrats on your invites.
You guys are going for an Assessment Centre,it's no interview.
You would be grouped and given a material to read and have a simulated board meeting with your group where you would try to come up with workable solutions. You have to notice the salient points relevant to drive the discussion. You will find that there are lots of smart people so you have to bring your A game. You do not need to force your opinion or try to moderate,you have not been made a leader. If you are articulate and have a proper sense of timing,you would see that your colleagues may defer to you.
The assessors would be there while you discuss. Pay them no mind,do this exercise like you were in a real board meeting.
At the end of the group discussion,you would be given individual tasks that require you to make a 15 minute presentation to a panel of assessors using flip charts. You will find that your O'level Economics didn't go to waste.
I wish you all good success.