“Gettin’ the Band Back Together” is a genuinely funny new guilty pleasure of a musical with a tried-and-true formulaic structure and a host of scrappy, likable characters; the score is unfortunately unremarkable, derivative, and forgettable, but the book, developed through improv rehearsals, is sitcom-light, irreverent, well-beat, and really silly.  Others may pile on, but I must admit: I had fun.