Complex sentence: Martin became the last member to join the legendary group and apart from Stephen Stills and Richie Furay was the only musician to stay with the group from its inception in April 1966 to its disbandment on May 5,1968.
Split into simpler sentences: Martin became the last member to join the legendary group at its founding. Along with Stephen Stills and Richie Furay, he was one of only three musicians to stay with the group from its inception in April 1966 to its disbandment on May 5,1968.