Not old enough to be in the First Avenue crowd for the "Purple Rain" finale? Weren't lucky enough to be Courtney Cox when The Boss shot his "Dancing in the Dark" video here? Here's your chance to be a Hollywood footnote: The "Prairie Home Companion" movie wants extras tomorrow, Friday, July 8. PHC needs about 850 adults, no kids, to fill the seats at the Fitzgerald Theater at 11 a.m. For full information call 651-224-7541. And bring a sandwich for Lindsay Lohan.