Complex sentence: They ended in a club and watched a pair of musicians who by Reich's account were terrible guitarists and singers, however when they started clapping very loudly, Reich and his group, who were mainly percussionists, joined in.
Split into simpler sentences: They ended in a club and watched a pair of musicians who by Reich's account were terrible guitarists and singers. However when they started clapping very loudly, Reich and his group, who were mainly percussionists, joined in.