The Society address the letters "Dear Brothers:" but that probably wouldn't be appropriate for you if you don't consider them to be your "broithers" anymore.
It depends on the purpose of the letter, and also your "status" (already DF'd or DA'd, in good standing, not even a member, etc)
You could start it with "Gentlemen:" or "Body of Elders:"
When I wrote my DA letter - which was actually a "stay off my property" letter since I had stopped associating years earlier - I addressed it to an elder's house because that's where the mail goes. (The Hall has no mailbox. All mail goes to an elder's home.) So the envelope said
XXX Congregation
[elder's name]
123 Main St,
[town], FL 00000
And I started the letter
"Congregation Elders:"
I closed the letter with
"Respectfully,
[my name]"
because I was not being confrontational; I just wanted to tell them bascially, "Please stop coming on my property and leaving literature because I quit this religion years ago. Just remove my from your records and leave me alone."