Hi again!
To help you a little more with what will take place I could tell you how was my assesment day in brief.
First of all regarding the group exercises I'm not sure if they still do it as some people after August 2006 told me that they had not done that.
Nevertheless two different exercises,first one a village is about to flood and the group's responsibility is to save everybody on it.Each participant has a list describing the available means for the job e.g one has a bus another one has a boat etc.So putting everything on the table you have to come out with a plan within the specified amount of time(10 min?i really don't remember)and save everybody.
Second one you're given a set of toys which you have to construct them in a specified way,what a disaster....I didn't understand a thing(Even the guys that English was their mother tongue had problems understanding what was required!!!)
In general I would say that don't be really stressed with the group exercise as if you go well with the technical exam and the interview you'll most propably be succesful on day one.
Interview now

escribe your worst day at work,describe a day when you gave a solution to a problem that arised in the cockpit,a day you flew with a crew member who did not have good CRM and what you did about it,how ezy could benefit from you.
Also is agood idea to refresh a little bit the metars especially the ones with RVR and if type rated go through the systems.
Best of luck colleague