I did a group interview a few weeks ago - it was very casual though, at Gloria Jeans! Anyway, there were three people in the interview (and two interviewers) and basically you just have to show how well you communicate, how you work with others and your personality. The interviewers took time to ask a question then listened to what we each had to say. I don't think they were necessarily harder than single interviews but you need to be able to talk about yourself. I guess the larger the group the harder it would be to stand out (unless you're one of those super-confident, in-your-face type
)
Basically:
* be yourself - don't try too hard to impress, it's good to be outgoing but don't go over the top
* speak clearly and not too fast, so they can understand you
* dress professionally - don't show up in jeans and a t-shirt
* don't interrupt others. If you want to say something wait until others have finished speaking
* say hello to the other applicants (if you get there early, for example) - show that you are friendly and outgoing