Complex sentence: According to Charles K. Johnson the membership of the group rose to 3500 under his leadership, but began to decline after a fire at the house of Charles K. Johnson which destroyed all of the records and contacts of members of the Society.
Split into simpler sentences: According to Charles K. Johnson the membership of the group rose to 3500 under his leadership. The organization was hurt by a fire at the house of Charles K. Johnson which destroyed all of the records and contacts of members of the Society.