Came upon this while reading a blog posted by Hikasa Youko. Apparently herself and 4 more seiyuus are forming a group together, like Sphere because of the "Ro-Kyu-Bu" anime series that's airing soon. The members are as follows: Hanazawa Kana, Hikasa Youko, Iguchi Yuka, Ogura Yui, and Hidaka Rina.
Ogura Yui (15) is an idol whose movements were digitized for Hatsune Miku's dancing in Project Diva. She has been in several TV dramas, and is beginning a career as a seiyuu, having been Ringo and Mint in Yumeiro Patissiere.
Hidaka Rina (16) is an actress in TV dramas who has become a seiyuu, most notably as Last Order in Index.
Hikasa Youko is Mio in K-On. Iguchi Yuka is Index in Index. Hanazawa Kana is Nessa in Fractale, and was Tsukimi in Kuragehime, etc., etc.