I think I need to change where I shop, worth the 10 minute drive across town?
But, back on topic...I just had a group interview for a 'civil servant' position and there were five of us in a group and we did three different activities. Two of them were more teamwork based so were looking at how you work with others and the last one was a group discussion.
Nick pretty much hit it on the head, put your opinion over but also listen to others and be proactive if there is any teamwork related activities.
Oh and don't stress about it on the day, try and have some fun and it will help you be more outgoing and less of a wall flower!