Adding additional users is easy. In the users section, click on Invite User on the top right corner, and enter new user's email address.
The invited user will get an email with an invitation link. Once the invited user accepts the invitation, by clicking on the provided link, (s)he will be able to set the password.
By default, the newly invited user will only get permission to edit content and add/remove devices.
To grant permission to other areas, like modifying billing information, adding new subscriptions, etc., the original user should go back to the users section, and update the permission to "Administrator".