Complex sentence: Glan's first serious band was the Canadian soul band The Rogues (later called Mandala) which he formed with keyboardist Josef Chirowski and bassist Don Elliot; they had worked together in other bands like Whitey & The Roulettes.
Split into simpler sentences: Glan's first serious band was the Canadian soul band The Rogues (later called Mandala) which he formed with keyboardist Josef Chirowski and bassist Don Elliot. At the time they had already played together in other bands like Whitey & The Roulettes.