@BrianH phil,
This being the Internet, any of us, including me, might well not exist - at least in the sense of possibly being a false identity adopted by someone who
does exist - even if it is unlikely that any of us is actually an algorithm passing the Turing test. (In some cases, mind you, I don't know ...)
However, you
know that
some people, other than yourself, exist.
Also, I know that you know that some people other than yourself exist.
(Assuming that you exist! - And if you don't exist, none of what I say matters, so I might as well go on saying whatever I want, regardless.)
Also, you know that I know - or, even if I do not exist, you know that any
really existing person reading this thread knows - that you know that some people other than yourself exist.
So you can't bluff me (or anyone else who does exist, even if I do not exist).
So I don't have to prove to you that some people other than yourself exist. It's an axiom, which you already accept.
If you address yourself to any of those people whom you already know to exist, then no proof of their existence is needed.
On the other hand, if you don't so address yourself, then there is nothing meaningful to be proved.
However, it is possible that anyone, even all the people whom you know to exist, including yourself, might not be whom they appear to be, nor might they be whom they take themselves to be.