A majority of the members in After School have been back-up dancers for Son Dam-bi as well as for other artists before debut. Park Ga-hee, After School's leader, has been seen as a back-up dancer for artists under both SM Entertainment and YG Entertainment, most notably for BoA. Park was also part of a Korean American girl group called S.Blush, whose first digital single peaked at #2 at the Billboard Hot Dance Chart. In 2008, Park was featured in Son Dam-bi's "Bad Boy" music video as a rapper, and Kim Jung-ah was featured in Son's performances as a back-up dancer. Lee Ju-yeon was featured in Son's "Crazy" performances. Days before After School's debut, Pledis Entertainment officially announced that the group's music concept is heavily influenced by the Pussycat Dolls, and that they "will be a type of group that Korea has never seen before." On January 15, 2009, Sony BMG released a 30-second teaser video to the web, reaching 10 million views on the first day. The girls, as a group, made an unofficial first appearance on December 29, 2008 at the SBS Song Festival 'S', performing "Play Girl" with Son Dam-bi.