Complex sentence: Guitarist Stephen Fitzpatrick has been with the band since 1999, drummer Kellii Scott intermittently since 1999, and bassist Nicole Fiorentino since 2006.
Split into simpler sentences: Guitarist Stephen Fitzpatrick has been with the band since 1999 and drummer Kellii Scott has worked with the group on and off since 1999. Bassist Nicole Fiorentino has been with the band since 2006.