Group Assessment advice?

I have recently been invited for a group assessment, after a successful first round interview. From what I was told, a group assessment involves getting placed into groups and completing a given project to complete in the required time while interviewers watch you and take notes. I would assume they would be marking you on your teamwork, leadership skills, listening skills, communication skills, etc...?

Any tips? How should you react in various situations? Eg: when you have an irrational group member, a slacker, a dictator, an idiot, etc.

Thanks for your help!