Usernames are unique; a new user cannot be added with an existing Username. When this occurs during the registration, the following message will be displayed and the user must proceed to select a different username.

In order to add a new user the administrator, when logged in, has to click the User Accounts button in the menu.

Click Add New User.
Complete all form fields.
If the Authorized box is checked, the user will automatically assume the registered user role and can login in the portal. If it is not checked the user will not be authorized to any security role.
Click Update. The user will be displayed in the list.