I am not certain what event you mean. If you mean the Ironbutt Rally it is held every two years and is a lottery. I think you have to be a IBA member to enter but not sure.
To become a member you just have to complete a ride. Most complete the "easiest" ride first, which is the Saddle Sore 1000 (at least 1000 miles in 24 hours or less). It is easy to under estimate the difficulty of this ride. After my first SS1000 in 1997 I could barely walk.
Go to Ironbutt Forums. Even as a non IBA member there is a forum for advice to "future members". You can ask questions and get answers from lots of IBA folks.
At the request of some others I created a non-official site to help would be riders with documentation and such for IBA rides, particularly the SaddleSore 1000.
FYI be sure to be meticulous with your gas receipts. I put a number on each one in chronological order. You need to ride on you sheet the corresponding number mileage, location twon...state, time ....hotel receipts for more than one day trips, The harder the ride the more they will scrutinize it....