Who's who, is an expression referring to the most outstanding best known young individuals in Hollywood. The most popular young Stars of Hollywood

The phrase "who's who" is used to refer to a list or group of famous, notable, or important people. For example, a who's who of the tennis world would probably include famous players such as Rafael Nadal, Roger Federer, and Novak Djokovic. In your specific example, "a who's who of young Hollywood" means something like "a list of the most important young people in Hollywood". Overall, the sentence could be rewritten as something like: "The cast of 'The Last Summer' is basically a list of the most notable young actors in Hollywood." Hopefully that helps!

Who's who of young Hollywood = the most famous / important young actors of Hollywood.