Complex sentence: Both Gibson and Miller served as vocalists and guitarists in the Gibson/Miller Band, which also comprised Mike Daly (steel guitar), Bryan Grassmeyer (bass guitar), and Steve Grossman (drums).
Split into simpler sentences: Both Gibson and Miller served as vocalists and guitarists in the band. They were backed by Mike Daly (steel guitar), Bryan Grassmeyer (bass guitar), and Steve Grossman (drums).