Pentatonix: Friendships and harmonies

The a cappella quintet Pentatonix has won three Grammy Awards and sold 10 million albums, while their YouTube videos have been streamed more than six billion times. David Pogue talks with group members Kirstin Maldonado, Scott Hoying, Mitch Grassi, Kevin Olusola and Matt Sallee about bringing their heavenly voices to the sounds of Christmas – and about introducing musical instruments to the mix.