welcome to mallvibes! it's great to see more males joining...
.
i have never worked in retail before, but i have had group interviews. i am pretty sure the point of having a group interview is to see if you are a team player, also if you can take intiative and/or help out when needed. it's about showing that you can work independantly as well with other people (since guess is not a one person show). I don't know what kind of questions they would ask, but they might give you some senarios related to customers, employees stealing, etc... Might list your strengths, and weaknesses. I think you should not dress too casually, because this is an interview. I also think if you have guess clothing you should wear that (or nothing with other namebrands/logo's on it).

goodluck, and keep us post-dated!