You can't call Isis an extra, to say nothing of being "just there to fill out the group." This was not a crowd scene. She has a character name, she gets a close-up and a medium shot just of her, a smashing costume unique to her, and she plays a scene with Teri Garr. She's just a tiny step below the unfortunate cabaret dancer in "Wolf in the Fold," in that she got no lines. They'd have to pay her more if she spoke.
If only they'd listed her in the credits!