To add a new user, go to Manager, Staff and choose Edit Staff Members

You will see the Manage Staff Members window where you can Edit, Delete and Add users:

When you click on Add user button you will see another window, Edit User. In the User Details part, it is required to enter the e-mail, the first name and the last name of the user. On the right side choose the role of the user.

Save the changes. At this point, the system will send an automatic message with the password to the email of the user so that they can access their account.

If the email has no password, you can reset it and the system will send you a new one, see here: 

How to reset the password

See also: 

Setting up roles

How to reset user's password?

How to change user's password?