I think the whole front two rows (the girls in matching outfits) are the girls that have been on the show as regulars since early on (ie, THE Possible, Affilia Saga East, the anime head girl, etc), and the girls in the back are all girls that auditioned for the Backstage Pass (and presumably passed the audition?). Certainly everyone that tried out didn't get in, because audition numbers went into the several hundred at least?
I'm sure this article addresses it somewhat, but I'm lazy/don't know Japanese.
http://www.oricon.co.jp/news/entertainm ... ?cat_id=tw