As a moderator on https://raspberrypi.stackexchange.com/, I created a private chat room for the purpose of super-pinging a user to discuss some things.
After I created the room, I tried to add the user under "Enable Write Access", which says:
enter the user id or the user's profile URL
But adding the user's profile URL does not seem to do anything. It does not display "User does not exist" or any other error message, but the user is simply not added. Once I tracked down the correct ID, I was able to add the user that way.
I have since replicated this by randomly trying to add other users by profile URL -- both the base page, and the actual "Profile" link which has a
?tab=profile query string. Neither of these work, nor do they show any error.