Recently I received a question from a CaseInterview.com student about how to prepare for a 2nd round interview with Monitor Deloitte as well as how to prepare for a group interview (this student was told that their 2nd round interview would also be in the group format).
To prepare for the group interview format specifically, you need to keep in mind that it tests similar skills as the 1:1 case interview (which is why perfecting your core case interview skills is important), but in a different format.
However, in the group format you are being tested on your people skills much more intensely than a 1:1 interview.
The big thing to remember is don't be a jerk. Don't put down other candidates and say their idea was dumb -- thinking it will make the other person look bad and by comparison you look good. This will backfire.
Trust me, if someone else in your group says something dumb, the interviewers will know it. You do not need to point it out. And if you point it out in a disrespectful way it will reflect extremely poorly on you.
Be respectful with everyone and be a straight shooter.
If you were wrong, say you were wrong (in a confident way).
If someone else comes up with better idea than yours, say forget my idea... yours is better. Being flexible is a desirable trait. Being stubborn in holding to your point of view in the face of clear data to the contrary is a liability.
The key is to forget it is an interview, and pretend you are working on solving a client situation and you should be fine.
The "goal" is not to beat everyone else in the group.
The goal is to help the group succeed.
If the group consists of people who meander analytically -- can't form a hypothesis, can't come up with an issue tree, can't come up with MECE issue tree, can't figure out what data would be needed to test a hypothesis -- then take the lead and help the rest of the team through the process... but do not leave them behind. Instead, bring them along.
If they are good at the process, then work with the rest of the team to advance the joint problem-solving process. The goal is to demonstrate you can do the job and you are someone nice enough that the interviewers would want to work with you. The goal is not to make everyone else look bad or to "beat" them.