The noun phrase modified by the prepositional phrase "at the movie's premiere" is "Sideshow Mel's friends".
"Last night" is a prepositional phrase that modifies the verb "were". "Sideshow Mel's friends" is a noun phrase that refers to the people who were not at the movie's premiere.
"weren't" is the past tense verb form of "to be" in the negative form. "at the movie's premiere" is a prepositional phrase that modifies the noun phrase "Sideshow Mel's friends". It tells us where Sideshow Mel's friends were not.
Therefore, the noun phrase modified by the prepositional phrase "at the movie's premiere" is "Sideshow Mel's friends".