You need to register as a member which requires an auction access # and representative #, which you have to get either from being registered as a dealer or being employed as a salesman or inventory manager at a dealer. Anyways, to become a member at say Manheim Auctions, you need to be registered as a dealer with your state and then depending on the state, there's a license test to be a salesman.
Or you can go to craigslist and there's usually someone that is willing to take you to a dealer auction if you give them some money.