From the mission statement page on the FSA website:
Day-to-day business of the Florida Sheriffs Association is directed by a strategic plan created and approved by the 67 sheriffs of Florida.
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So, it seems like if you are a Sherrif in FL, you are a part of it.
I'd imagine the agenda is steered heavily by the 5 member board of directors (all sheriffs):
Sheriff Sadie Darnell
President, Alachua County
Sheriff Jerry Demings
Vice President, Orange County
Sheriff Brad Steube
Secretary, Manatee County
Sheriff Mike Adkinson
Treasurer, Walton County
Sheriff David Shoar
Immediate Past President, St. Johns County
ETA: They also list their corporate sponsors on their site. Perhaps those corporations would like to know how disappointed we are that they are sponsoring such an anti-freedom organization. Adams Arms is on there as a bronze annual partner, for example.