According to Wikipedia : https://en.m.wikipedia.org/wiki/Iron_Butt_Association
The Iron Butt Association (IBA) is a US-based organization dedicated to endurance motorcycle riding with a claim of over 60,000 members worldwide who tout themselves as the “World’s Toughest Riders.” One of its more popular slogans is “The World Is Our Playground.”
The IBA does have membership in a traditional sense. There are monthly newsletters sent to subscribed members. There are no mandatory yearly dues, although members can voluntarily pay an annual fee to become a premiere member. Premiere members receive occasional news and information from the IBA staff, early notice of some events, additional chances to enter some events, discounted entry fees for some events, and access to special areas of the web-based discussion board.
The IBA is a loose-knit organization with only one way to earn membership: ride one of the several rides the IBA certifies.
All that sounds pretty accurate. You can check out the official IBA Websites here :
Iron Butt Association Web Page : http://www.ironbutt.com
Iron Butt Association UK Web Page : https://www.ironbutt.co.uk/w3/index.php
Iron Butt Association Ireland Web Page : http://www.ibaireland.org
It was this bit that got me : ‘with only one way to earn membership : ride one of the several rides the IBA Certifies.’