Do you write Hi all or Hi All?

Answer 1

You do not capitalize every word unless it is a title of a book. So you would write, "Hi all."

Answer 2

I dont ever write either. All is a common noun, and you should not capitalize it, unless its the first word of a sentence.

Answer 3

It is more appropriate to write "Hi all" with a lowercase "a" as part of a formal greeting. This is because it is a standard convention in written communication.