Complex sentence: In their wake rose a new group called the Canadian Group of Painters, of which Group of Seven members included Lawren Harris, A. J. Casson, Arthur Lismer, A. Y. Jackson, and Franklin Carmichael.
Split into simpler sentences: In their wake rose a new group of artists called the Canadian Group of Painters. Several of the Group of Seven painters were included in the new group including Lawren Harris, A. J. Casson, Arthur Lismer, A. Y. Jackson, and Franklin Carmichael.