Complex sentence: The genteel First Club of Boston would have 107 members by the end of 1889--including author Edward Everett Hale, magazine editor William Dean Howells, and prominent socialist writer Laurence Gronlund--and would dominate the initial phase of the American Nationalist movement.
Split into simpler sentences: The genteel First Club of Boston would have 107 members by the end of 1889, adding to its ranks author Edward Everett Hale, magazine editor William Dean Howells, and prominent socialist writer Laurence Gronlund. The group would dominate the initial phase of the American Nationalist movement.