Complex sentence: He contacted his childhood friend from summer camp, Eli Gerstner, who was by then a successful producer, together they gathered the choir and started working on the first album (Kol Hamispallel).
Split into simpler sentences: He contacted his childhood friend from summer camp, Eli Gerstner, who was by then a successful producer. Together they gathered the choir and started working on the first album (Kol Hamispallel).