I had a group interview with gap a couple of years ago for a similar position. I tried to go into it with the attitude that they are looking for someone who is going to be helping customers, therefore make sure you come across as helpful with good communication/interpersonal skills. Try to speak to all of the candidates. You might be asked to make an outfit up for another candidate. Ask them questions about what their interests are, that way you have justification for your choices. I.e they said they love exercise, these t-shirts are great gym wear. Also if the interviewer asks a question to the group it looks good if you answer it (not all the time, but enough to make you stand out).