Forwarding between two domains pointed to the same directory of the server without changing the URL

RewriteEngine On
RewriteCond %{HTTP_HOST} ^domain2.com
RewriteRule ^(.*) https://domain1.sa/users/John/$1 [P]

It's not clear why this would result in a 500 error, unless mod_proxy or mod_rewrite is not installed or configured properly? Or you have a conflict with other directives perhaps? You should check your error log for the specifics of the error.

However, if the two domains point to the same server/directory and you wish to keep domain2.com (the original URL) in the browser's address bar, then you shouldn't have to proxy the request (as you are trying to do here). A simpler internal rewrite (on the same domain) would seem to be all you require. For example:

RewriteEngine On
RewriteCond %{HTTP_HOST} ^domain2.com
RewriteRule !^users /users/John%{REQUEST_URI} [L]

The regex !^users is to avoid a rewrite-loop (which will also manifest itself as a 500 error in the browser).

So, instead of typing domain1.com/users/John, I will type domain2.com.

But note that the above will send the request to /users/John/ (with a trailing slash), not /users/John (no trailing slash) as you've stated here - is that an issue?

And you are wanting to rewrite from /<something> to /users/John/<something> - as per the code sample you posted? Although this is not mentioned in your question text? If you only want to rewrite requests for the domain itself, ie. domain2.com/ and not domain2.com/<something> then the above can be simplified further. For example:

RewriteCond %{HTTP_HOST} ^domain2.com
RewriteRule ^$ /users/John [L]