h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Speaking as a linguist and a native user of the pronoun in question,
I tend to pay more attention to its use than most people. The entry
is mostly right, but there are a few problems. Some people suggest
that "y'all" can be used with singular reference, but having travelled
throughout most of the South I have never experienced anyone who
uses it this way. "Y'all" can, indeed, be used in the presence of
a single person, or to a single person, but that person must notionally
be a member of a larger group. So, for example, if one happens to be
in a restaurant and asks to a single waiter "What kind of drinks do
y'all have?", the reference is to the waiter in his or her capacity as
a representative of the restaurant, and is asking about the drinks
the restaurant serves, not the waiter's own personal collection. Even
native speakers who are not careful often confuse this usage.

Now, it's true that when you modify it with "all", one must use
"y'all" -- "all of y'all", or "all y'all". (In fact, that is evidence
that the word is not a contraction at all, but has now gone on to
become a full-fledged pronoun.) Saying "all of you" would sound distinctly odd in normal every-day speech.