Complex sentence: The cast is forbidden to perform by their union, so Blitzstein takes the stage alone at an upright piano to perform the show himself, only to be joined by many of the cast members who deliver their lines from the audience.
Split into simpler sentences: The cast is forbidden to perform by their union, so Blitzstein takes the stage alone at an upright piano to perform the show himself. He is shortly joined by many of the other cast members, delivering their lines from the audience.