The group remained a trio, mostly singing together for fun, until they got the idea to audition for "The Sing-Off," NBC's singing competition for a capella groups. While they were already strong as three, they decided they'd do better in the competition if they had two more voices. They met Avi Kaplan, who became the band's bass singer, through a mutual friend.

With the fourth member secured, they just wanted a beatboxer to finish rounding out their sound. Around the same time, Kevin "K.O." Olusola released a video of himself beatboxing and playing the cello. Kirstin Maldonado said it felt like fate when his video went viral while they were on the hunt for their fifth member. "We just kind of stumbled across him and thought he was so amazing and so cool," she told Sweety High.

After K.O. agreed to join, they had the full group, but they were all in different locations. Although Maldonado had grown up singing with Hoying and Grassi, the full band didn't get to rehearse together until the day before they planned to audition for "The Sing-Off." Fortunately, in Maldonado's words, "The second we all sang together as a five-person group, it literally was magical." She continued, "it honestly was just amazing how well it flowed together [...] We had no idea, it could have been horrible," she said. They named the group Pentatonix, and the band was born.