Here is a link to the User Manual, maybe it will help you.
Have you read the _ Help _ provided in the dashboard? The following is copied from there.
Role of Users in Your Personal Cloud
Everyone who accesses your personal cloud must have a user account for the WD My Cloud drive. As the WD My Cloud owner, you have a special user account (admin) that gives you administrator privileges. With these privileges, you can set up and configure the drive to your specific needs, as well as add other people to your personal cloud. You have the power to determine exactly what other users can access on the drive.
As a default, the admin user name does not have a password. You can add one anytime.
Important: When initially setting up your drive, you have the opportunity to change the administrator name and set an optional password while running the WD My Cloud setup software or completing the Getting Started screens. Make sure you keep track of the name and password. You won’t be able to display the WD My Cloud web interface without those credentials.