Overall, you want to approach the group case interview as follows. It's your team versus the case. It is NOT you versus your team mates.
If one of group says something where you disagree, make sure you remain very composed and professional. You should also NOT try to be the leader. Just be very clear and structured in your contributions, stay open for compromise. Trying to look especially smart is the first way to get dinged - believe me!
Just trust that your case skills will shine through naturally without you having to worry about the competitive aspects of the situation.