Are there any secret or unknown celebrities?
No. If they were secret or unknown, they wouldn't be celebrities.
Since the Illuminati are not real, the safe answer is that no celebrities are members of it. This "secret society" seems to have existed in some form back in the 1700s (some people associate it with Freemasonry); but contrary to many internet rumors, there is no credible evidence that any such organization is in existence today.