I guess this is an easy question to answer when you have only one founder. It was Ossian Everett Mills, the Father of Sinfonia, who had the idea to bring together "old boys" and "new boys" at the New England Conservatory for what became the Sinfonia Club and, within two years, the Sinfonia Fraternity. Mills established the principles and values on which the Fraternity was built and was a guiding force in our first decades. He served as the first and fourth Supreme President and was elected "Honorary Grand Supreme President for Life" in 1904.
The 13 men who answered Mills' call to form the Sinfonia Club are referred to as the Charter Members.
Interesting aside: Mills' grandson, Arthur Mills, was initiated into our Alpha Alpha (national honorary) chapter in appreciation for his assistance in historical research efforts in preperation for our Centennial (1998). He was in his 70s at the time. In 2008, one of Mills' great-great-grandsons, Eric Mills, was initiated into Gamma Theta Chapter (U of N Texas); Eric's father was later initiated as an honorary member of that chapter.