Good luck! First of all, relax! Retail stores will often hire those who are unemployed, especially because it means they can train them in to act exactly how they want them to act, not how they were trained by other stores. Make sure you're incredibly out-going and friendly. Dress in Forever 21 clothes if you can - don't go too simply, but don't be a fashion victim either. A good thing to wear is a dress with tights and brogues, or shorts and tights with a top. Do NOT wear a suit or anything like that, your pieces of clothing don't really matter but just make sure they're trend-aware and fashionable, not stuffy or too conservative, Forever 21 clothes are all about affordable, trendy pieces you WANT to wear.
Seeing as how it's a group interview, they will need to notice you (for the right reasons), so try to just talk as much as you can and say things that the others haven't already said - but don't try talk them down either.
You'll likely be asked questions like - who is a fashion icon you look up to and why? What was something you noticed from the Forever 21 store you recently visited? What are the qualities of a good sales associate?
At some point, they're likely to ask you what you know about the store. Try to provide a little more unusual details - things like who set it up (Changs), brands within the store, etc.
They'll want you to be bubbly and happy. Be professional, obviously, but don't be boring or guarded. Top tip - no matter WHAT you do, bring it back to customer service, it is ALL they care about. If they give you scenarios about something happening in the shop, make sure you prioritise the customer and providing excellent customer service. Say how much you love the idea of bringing american style customer service to the UK, and how that's something different the store does over its competitors. I cannot stress this enough! They have an acronym for FOREVER which is to do with customer service - make sure you're giving the customer as best an experience as possible, while trying to sell more for the company (for example, if you help a customer pick out jeans, then also suggest a top that would look great with it. If they're looking at lingerie, make sure they know about the lingerie promotions they have on an ongoing basis)
How do I know exactly what they want? I'll leave it to you to guess
EDITING FOR OUTFIT EXAMPLES:
http://www.forever21.com/images/model_front/00048354-02.jpg this dress, with black tights and brogues.
http://www.forever21.com/EU/Product/Product.aspx?BR=f21&Category=top_blouse_shirts&ProductID=2019572268&VariantID= this blouse, with black ankle skinny jeans and pumps