Domain names in AMPR.ORG are available to any licensed amateur radio operator who is interested in advancing the art of ham radio digital communications. In most countries, there is a local coordinator who is responsible for assigning an address and updating the master hosts list. https://portal.ampr.org/networks.php There is no formal organization, no membership requirements, and no dues. All of the work is done by volunteers as they have time to do it. Note that an IP address must be associated with a domain name in AMPR.ORG in order for the main gateway at UCSD to pass packets to it.
If you are a licensed ham who needs an address, contact your local coordinator. AMPR.ORG is not intended to be, and should not be used as, a substitute for buying a personal domain name.