Is it even grammatical? A little bit of Googling returns quite a number of searches which lead me to think it is a grammatical sentence.
All the links you cite should have been either "Tell us what you think" or "Let us know what you think" (one of them even has the correct title, but is wrong in the first sentence). Getting confused, and ending up halfway between the two, is not uncommon, but it is not right. (In my opinion, it never will be right no matter how often it is used: but it is definitely wrong now).
This is an error. Probably the writers of this phrase have in mind the common phrases
tell us what you think
let us know what you think
and simply mix them up.
“Tell us know what you think” is not grammatical. The first three words are the problem: You can’t do “VP VP” in English (VP=Verb Phrase). The Google link shows lots of instances, but it is not at all grammatical.
Most likely it is a very easy typo to mistakenly write “know” for “now”.
The following sentence is probably what was intended in all those cases.
Tell us now what you think.
It is not grammatical as written; assuming it’s not an internet meme of some sort, I can only presume that it was intended to be “Let us know what you think”, but somebody typo’ed “Let” into “Tel” and then the spell-checker corrected from “Tel” to “Tell”.
Tell us know what you think
Let us know what you think ←
Tell us now what you think ←
Let us now what you think
The second and the third ones are meaningful.