Hi,
I'm in my first year at Sussex. The interview will mainly discuss your application and reasons for wanting to join the course, I was asked about a relationship that gave me strength and one that was a strain.
The group work is pretty easy and nothing to worry about, you are given the details of the discussion when you arrive and will have plenty of time to read it and think about your answer/view on it, you will then sit with a few applicants and discuss it together. Your be watched by a tutor while doing so and they will take notes about how you interact with each other and how you put your views across and willingness to consider others.
Just relax and enjoy the experience, I'm yet to meet a tutor, carer or professional within the Uni that isn't approachable or helpful..... they are all lovely!