The likelihood that a genuine organization resembling the legendary Illuminati exists is about nil. It probably doesn't exist, and probably never did.
There are groups that exist in the modern world that may present themselves as an off-shoot of the Illuminati or even as the remnants of that organization today, but these claims should be viewed with very great skepticism.
It's unlikely that the Illuminati we understand and know of from pop culture works of the modern era ever actually existed, and even if it did it would have ended long ago.
The simple answer to your question is likely, "You can't."
If it never existed, there would obviously be no way to contact them.
If they existed once but don't any more, then there's obviously no way to contact them.
If they exist and are real, then they're a secret society that by definition is so secret, you cannot contact them.
Regardless of whether the Illuminati actually ever existed or exists today, the answer is still the same, "You probably cannot contact them."
Place your questions in a bottle and toss it in the ocean.
No, wait, scream your request from the northernmost rim of the Grand Canyon.
Hold on, that's not it either. Oh! I know, the Illumati doesn't exist, so you can't contact them.
Illuminati is Latin. You just say, "Illuminati."
No. No one is Illuminati. The Illuminati does not exist.
No. There is no Illuminati. The Illuminati was a fraternal organization in 18th century Bavaria.
Ariana Grande is not a member of the Illuminati. The Illuminati is not real.
No, she is not Illuminati.
Again, there are no members of the Illuminati that are in show business.
No. No one today is Illuminati. The Illuminati have not existed since 1785.
No, Trey Songz is not an Illuminati. The Illuminati is not an organization of entertainers or celebrities.
The real name for the organization Illuminati is precisely that, the Illuminati.
All of them. No singers are affiliated with the two hundred plus year defunt Illuminati. The Illuminati does not exist.
Illuminati is not a language.
There is no such thing as the illuminati.