Attempted to add a Mapper authenticated user both before and after new user registered and validated new email address. New user unable to log in due to “invalid username/password”, password reset attempt gives “unable to use this email address” failure. (Note that I used an alternate email of my own to test this feature in case that matters.)
In Mapper System/(station)/Settings, new user appears OK after ADD, but is not visible after Settings closed, then reopened.
Is there a delay between new user registration and allowing the ADD?
Is this feature actually implemented yet? 
Did you get a verify Email for the second user? If the user is not verified by email you can not reset the password or login, This seems to be the case i guess, at least it sounds like it.
You can send me the email of the new user you created via PN and i will have a quick look.
regards,
Manuel