Complex sentence: Warren had not in fact previously considered doing so, but at the mere suggestion, he immediately began composing original songs and lyrics and teaching them to the aforementioned vocal group, which began to perform them publicly.
Split into simpler sentences: Warren had not in fact previously considered doing so, but at the mere suggestion, he immediately began composing original songs and lyrics. He began teaching the new songs to the aforementioned vocal group, who began to perform them publicly.