Currently free registration on Demonoid only opens once a month, although in May, registration was open twice for 2 days, on May 2nd and 8th respectively. Everyday else in the month registration will be closed, and interested users will have to find invites or invite codes which cannot be sell or trade from existing Demonoid users who are in good standing during these closed registration period.
Register for your free Demonoid account
http://www.demonoid.com/register.php (now demonoid.pw).
Missing out on the open registration period of Demonoid? Download Tracker Checker that monitor Demonoid and notify you when registration is opened.
Update: Demonoid public registration opens again on mid June 2007.