Group exercises can be tricky (and pretty much guaranteed to be artificial). You don't want to be a wallflower but you also don't want to be arrogant. I suppose 'straightforward confidence' is best, ie, don't shout down the others but also be sure to make your opinion heard. Sometimes, if possible, it can also be good if when you have the floor, you're able to direct the conversation to other people who have been quiet by asking for their opinion on the issue.
Basically, everyone is out for him/herself, naturally, but doesn't want to look bad in doing so. That's not to say they aren't nice people, it is just that it is such a competitive environment.
Also, I would recommend against being the person who takes notes (if you can do so without being obvious about it) - some firms consider that good for the group but not for you as an individual.

Ridiculous, I know. But best to be aware of it, I guess, so as not to give a firm even the slightest excuse to reject you. If you do get stuck doing it (because it is not like you can say you don't want to - it may be that you're in the wrong place at the wrong time), just try your best not to get sidelined from the discussion.