Depending on when you installed Scalar, your install may already have a registration key. For example, if you installed Scalar using Reclaim Hosting's single-click installer, Installatron, then a registration key might have been created automatically. You will know if a registration key is present if the Register key field is visible on the registration page.
Below are scenarios that involve the registration key and managing system users. They assume that you are a system administrator (a so-called "super admin"). Only system administrators have access to special admin tabs in the Dashboard and also the filesystem that Scalar is being hosted on, and the instructions below depend on the ability to access these administrative areas.
If your install has a registration key you can find out what the registration key is by heading to the Dashboard then clicking the All Users tab. (The All Users tab is present in the Dashboard if you are a system administrator.) On the right side of this panel will be a button: Show registration key. This key can be passed to users that you want to register on your install.
If you would like to add or change a registration key, FTP in to your hosting server then follow the instructions here: Managing a custom Scalar install with local_settings.php.
If your site includes a registration key and you would like to add new users without fussing with sending the key around, you can add new users yourself as a system administrator. Simply head to the All Users tab and fill out the Add new user fields at the top of the panel. You can similarly add new books in the All Books tab and other administrative functions in the Tools tab.