How does one get a Demonoid Invitation Code?

If you send me your email address I'll let you know when I spot it open next. You will struggle to find someone that will give you an invite code that works unless they know you. If you get warned or banned, they could too. Your best option would be to sign up when they open their doors to the public. The last time I spotted it open was for 5 days from the 19th - the 23rd of June inc., a shame you missed it. It may have been open since but I haven't noticed. Open Registration days can be any day of the week, are not necessarily every month and can last hours or days. I'd advise you to check the site EVERY day and eventually you will get there. If you can't get an invite you will need to follow this path, good luck. Please try and keep your share ratio above 1.1 when you get in. You may find this tool useful in the meantime, it's called Trackerchecker http://www.trackerchecker.org/ Sign in and select the trackers that you're interested in and you'll get an email when they open. Hope this helps.

You ask a friend who's already got an account, and he/she will send you one that can only be used once.