My www-data user had 644 permission to the config file, but I opened it up to 777 (temporarily) to test it, and it's still remains writeable by all users.
I also enabled groups, and didn't see it appear anywhere.
$config['globaladdressbook_perms'] = 0;
I am using the MyRoundCube Plugin manager if that matters.
I have the globaladdressbook plugin working correctly on our production email server right now, and have mirrored the permissions, file owners, etc, and still can't get it. Edit: Production mail server runs Apache. New dev server runs Nginx.
Thanks for your help