Let me all know what you think

Let me all know what you think.

In this sentence "ALL" is an optional element (vocative) used to reflect the person(s) addressed and related to ”YOU" appearing in the later half of the sentence. Words like this occur any where in the sentence and rather, fitting it to say, evaluating lebel of 'you'.

You are, darling, incorrigible.

In the poster, LET is used in a non standard way to infuse a kind of invitation/request as in the sentence below :

°Let me see.

ALL, appearing detached from 'you',becomes a noun itself and functions like 'darling' of the example above.

It conveys the meaning--all of you tell me what you think.

I don't know how such expressions sound to native ears but Indian languages abound in such usages.

It's completely wrong. Stop saying it.

Context provides what you need if you are doing this in person.

"Let me know what you think." (If you are addressing a group, it becomes a second person plural you by context).

"Let me know what you think." (Addressing an individual, it becomes a first person singular you).

Your ESL friend is having difficulty with the lack of a different word for second person plural from second person singular in English and your attempt to get around that using your own convention. (I get where you are coming from, having heard all kinds of variations on that over the years.)

Example from Spanish: Usted (you, singular) Ustedes (you, plural).

Some regional dialect in the US get around this using "y'all," but even that suffers from "is it singular or plural" as well. Is it 'y'all" or "all y'all" and so on. My dad is from near Pittsburgh, and "yuns" (youens?) crops up in his speech now and again as a second person pronoun. "Youse guys" wasn't uncommon to hear in the dark ages when I was young).

Simple solution for your phraseology problem?

Ask people "What do you think" and let the context determine singular or plural for 'you' in each case.