Yoshio was popular early in his career for being in a boy band, The Good-Bye. Their most popular song was "Kimagure One Way Boy". He's been in the industry for a while and I also don't have much of an idea how he joined Ayumi's band.

From what I gather, most of her staff have been with her for years, including Peco, who has been with Ayumi since at least 2001.