Hey, check out the previous two threads started by c4dm and I on GUESS interview,
GUESS interview
http://www.mallvibes.com/showthread.php?t=9477

Their interview tends to be very chilled (group interviews are usually even more chilled). So basically, just be yourself. For group interviews, you have to figure out how to stand out from the crowd in your appearance. An outfit inspired from men.style.com would definitely do in my opinion. Your style should be more important to them than wearing GUESS.

Group interviews are usually similar - the interviewer will have a list of behavioral questions such as "How would you deal with a difficult customer", "List an example of excellent customer service" (I swear, those two appear in every fashion retail interview I went) and more. Impress them, do not doubt them or stop smiling if they question the logic of your answer.

GUESS have been interviewing A LOT in these past two months. So don't be frustrated if you don't receive a call back. But good luck to you! Show them how guys like us roll =P