Just put in valid points and interrupt only when needed. Also prepare a good summary of the group discussion to say when your turn comes in the end.
I had prepared for heat transfer and thermodynamics. But they asked me on on Strength of Materials and Refrigeration.
Be thorough with the core subjects( especially Strength of Materials, if you are from Mechanical Engineering).
Be confident while answering the interview even if you don't remember the answer to a particular question.
Also in our case, the group discussion round was not an elimination round.
Also be thorough with your interns( you should remember what you did in your internships).