‘The Office’ star Jenna Fischer has joined the cast of Paramount Pictures’ new ‘Mean Girls’ movie musical based on the Broadway show. Jenna will play the come of Cady Heron, Mr. Heron. Other cast members previously announced include Angourie Rice, Reneé Rapp, Auli’i Cravalho, Jaquel Spivey, Tina Fey, Tim Meadows, Avantika, Christopher Briney, and Bebe…