The interviewers will evaluate you social skills and how well you work in a team. The goal during the group case is to find the right balance of not being too dominant, but also not too quiet. Make sure that everyone in the team has about an equal speaking part during the presentation.
Regarding the process: You will be in a group of 3-5 people with other candidates. First you will get the case question of a real client's case. You will have time to prepare and discuss with your team, take some notes to make a flip chart. At the end you have to present your results and recommendations in front of the “management board” (the interviewers will ask you questions pretending to be financial director, CEO etc).