Complex sentence: Kostroff is an active member of the Musical Theatre Guild, a Los Angeles-based theatre company that performs rarely-seen musicals, where he was part of the touring production of Les Misérables as Monsieur Thénardier.
Split into simpler sentences: Kostroff is an active member of the Musical Theatre Guild, a Los Angeles-based theatre company that performs rarely-seen musicals. Currently Kostroff is part of the touring production of Les Misérables as Monsieur Thénardier.