Complex sentence: He joined them shortly after their formation in 1964 and remained with them until 1984, when he left amid creative friction with guitarist Dave Davies, making him is the longest-serving member of the band, after the Davies brothers.
Split into simpler sentences: He joined them shortly after their formation in 1964 and remained with them until 1984, when he left amid creative friction with guitarist Dave Davies. He is the longest-serving member of the band, apart from the Davies brothers.