Optional usually means what it says - optional.  This Homeowners Association may have been established for the benefit of those that wish to use the facilities of the area, such as pools, tennis court, etc.  So membership is optional because if you do not want to use the facilities you do not have to join.  Check with the county and see if the HOA has files anything such as their charter or rules or that state agency that regulates them.  I am surprised that if you google them nothing comes up as many forums just love to blog about these things.  You can also run their name in the Court databases to see what pops up.  You never know. Good luck.