The diary is first introduced in "The Times They Are A Changin'" when Emily is seen writing in it in Hidalgo's. She received the diary from Kate, and later gives it to Richelle, who gives it to Giselle. Giselle then gave it back to Emily. After Emily and Riley appointed Skylar Dance Captain, Skylar received the book. Skylar is the current owner of the book.
It is originally said to be a secret diary, meaning that nobody other than the Dance Captains knows about it (which explains why Emily hides the diary when Riley enters Hidalgo's), although this account is later disproved when Giselle claims that she has heard about it.
Curiously, it seems as though Michelle never owned the diary during her time as Dance Captain — or if she did, the fact has never been mentioned on-screen. Neither has Amanda, although this may be because she was Dance Captain for only a short time.