It turns out the Illuminati have a website. Would I lie to you? It's right here.
The "About Us" section helpfully explains, "The Illuminati operates various departments and programs for the benefit of all people, in all places, from all generations. By protecting the interests of humanity as a whole, our organization has ensured the ongoing dominance of the human species over every other creature and predator on this planet."
You can join simply by giving them your email address; I thought it would be a little harder to get in. Screenshot above is from the email I got Tuesday.
I'm not sure how you figure out if it's the real Illuminati, but the group does have a press contact.
Via an article in the Independent by David Barnett about Freemasons. Barnett wrote to the press contact for the Illuminati, asking, (among other quesions), "can you let me know if all that stuff in the Robert Anton Wilson/Robert Shea books was true?"
No answer, alas.
Incidentally, the Illuminati's publicist is on Twitter, but the account is protected. Can't be too careful when you're a spokeswoman for the world's most powerful group!
Hat tip: @advantardeodus on Twitter.