Animal House!!
John Belushi
Tim Matheson
Tom Hulce
Stephen Furst
Bruce McGill
Mark Metcalf
Karen Allen
Kevin Bacon
(John Vernon and Donald Sutherland were already famous)
Revenge of the Nerds is also one of my favorites for this.
Robert Carradine
Anthony Edwards
Timothy Busfield
Curtis Armstrong
Ted McGinley
James Cromwell
John Goodman
Bernie Casey–already famous