All the FNZAS requires of a group is that it "has as its objects the assistance of those interested in aquarium or pond keeping or kindred interests".
I think you need a president and sec/treasurer at least as you need to be contactable as a club. To affiliate to the FNZAS you apply in writing to the FNZAS secretary. She then informs all other clubs of your wish to join the FNZAS. If there are no objections within 2 months your club is afforded full membership - on payment of capitation fees and Year Book levy. The capitation fee is $5 per person. If you already belong to another affiliated society you do not have to pay this sub twice. The YB levy is $25 per club per year. It helps go towards paying printing costs. If you can get a business to buy an advertising strip for $25 this can pay your levy for you at no cost to your members.
The YB lists all affiliated clubs and members with details of names, addresses, meeting times and places etc. It also details our trophies and competitions, club duties to the FNZAS, FNZAS duties to clubs, a list of retailers offering FNZAS member discount and all sorts of bits and pieces of helpful information. It also has our Constitution printed in it.