I've been assessed during a similar group assessment (we had to work together to produce a layout for a shopping centre), and also been an assesser for one of these excercises. All you are really trying to identify are:
1) The people that won't help others, and will intentionally try to throw other candidates under the bus (so you make sure you never hire them).
and
2) The people that do genuinely try to help the other candidates who are struggling (so you can hire them).
It's not exactly some sort of highly-complex psychological evaluation, you are just seeing how people work in a group task.