Membership is $74.99 plus sale tax annually per individual. At this time, registration can only be done online at either Disney.com/D23 or DisneyShopping.com. In the future, vouchers will be able to be purchased at the theme parks, the Disney Store and the Disney Cruise Lines.
International guests are able to join, but will have to pay additional shipping as required (for now, domestic shipping is free).
Gift memberships are also available to give to loved ones in addition to those under the age of 18.