Ironbutt Association
The Ironbutt Association promotes safe long distance riding. It has +75.000 members. Although it is a USA based organization it has members around the globe.
It has lots of challenges, the easiest and most popular is the Saddle Sore 1000 (SS1000), which consists in riding +1000 miles in under 24hs. During the ride some documentation has to be kept in order to “certify” the challenge. This is done to avoid cheating and keeping a high standard. For more information about how to certify a SS1000 here is the link.
The way to become a member is by riding a SS1000. Once a member, always a member. There is no regular fee to pay, there is an optional membership though which allows early access to events and some other perks.
I did my first SS1000 on 4/02/2016 and became member #63214