Complex sentence: In 1965, guitarist Pete Gage needed a singer to front his new band, and asked Washington to join and the latter was discharged from the U.S. Air force, he became the band's frontman.
Split into simpler sentences: In 1965, guitarist Pete Gage needed a singer to front his new band, so he asked Washington to join. When Washington was released/discharged from the US Air force, he became the band's frontman.