I've never been to a group interview or participated in one, but a friend of mine went to one at Bloomberg TV. She said that they sat around a table with various mucky-mucks from the company who all asked her questions.
In her case, it was a so-called "stress interview" where they fired business news questions at her and tried to trip her up to see how she dealt with stress. In yours, I would think it would be more like a one-on-one interview... but instead of you having to do this through a number of layers in the company-- they'll all be interviewing you at once.
Do your homework about the company. Do a google search in Google News. Be prepared, so you can wow them with your knowlege. Believe in yourself, and psyche yourself up! Remember, they're lucky to have you. Make sure they know that YOU are interviewing them as well!
Good luck!
P.S. What kind of job are you going for?