I don't have any experience with group interviews, but I'd advise you to ask questions. It shows that you are interested. Don't come off as an over-enthusiastic lick-arse, though. Employers like genuine people who seem honest and trustworthy. So it's important to relax and be yourself, and not to assume facades. A group interview is primarily a chance for the employer to introduce ye to the company guidelines. Consider beforehand how much you actually want this job. It's up to the interviewer to let you know what to expect from the job, so in a way, you are also an interviewer.
Be nice to your fellow wannabe-employees. Social skills are usually crucial in retail jobs, and the interviewer will probably monitor how you interact with your peers.
It's healthy to be a little nervous, but you should take this in your stride. You'd be surprised at how easy it is to fake confidence. Besides, it's only an interview, it only lasts a certain amount of time, and it'll be over before you know it. Worst case scenario: they don't call you back and you don't get the job. Big deal. Jobs are a very trivial part of life and you can always search for another. A better one.
Is your portfolio big? You might as well bring it. It'll show that you've prepared, and employers like people who bother to prepare. If you find that no one else has brought one, just put it on the floor or leave it outside or something.
Dress appropriately, have manners and all that jazz.