That site is not hosted on WordPress.com, so we have no control over who can or cannot access it.
The credentials for the new user are stored in your site’s database on your server. When you created that user at Users ->Add New there should have been a box you can check to email the new user about their account. That email would contain instructions on how to log into the site’s dashboard.
If they did not receive that, go to Users ->All Users and edit this user. Click the button at the bottom to regenerate their password, and send them their username and password via email. Then they can change the password again once they are logged in.
For more help with this, you can find support for your version of WordPress here: