Complex sentence: The only member of the original groups not participating in the new lineup was former Busted guitarist and vocalist Charlie Simpson, who instead opted to focus on his solo career and his band Fightstar.
Split into simpler sentences: The only member of the original groups not participating in the new lineup is former Busted guitarist and vocalist, Charlie Simpson. Simpson opted not to be a part of McBusted as he wished to focus on his solo career and his band Fightstar.