How to prepare for it
Group exercises are essentially a simulation of real work scenarios, where employers will measure your ability to work in a team; to lead, contribute, delegate, and to solve problems. They are also looking to assess your ability to lead your group to a good decision for the company. To do that, it helps to familiarize yourself with a few frameworks that can aid in decision making? Some are common frameworks like SWOT or the BCG Growth Matrix and sometimes you will have to create a simple score card system with scores for different ideas measured against key objectives decided by your group.
During the discussion stage, display initiative by volunteering to kick-start the process. Project confidence by taking the discussion forward without being too domineering. Be inclusive of opinions that are different from yours and do not appear defensive if your opinion is challenged. Be encouraging of every member in your team and if you notice any member getting ignored or sidelined, make it a point to engage them by asking for their opinion. Also remember that assessors can only evaluate based on what you say, so you have a great idea, verbalize it. In the process, your ability to listen to other people's ideas, be positive and seek consensus between dissenting opinions will be valued.
If someone has already taken a ‘leading role’ in facilitating the group discussion, do not try to take over his or her position. Instead, play the role of ‘best supporting actor’ by finding other ways to contribute. You can share idea, propose a useful decision-making framework, or help the facilitator bring the quieter ones into the discussion.