When you created your Sitebeam account, the login credentials you provided created the administrator login for your account. Administrators can add two types of users to their account. There are two types of users: builders and viewers.
Builders are users who can add websites and run reports. These are limited in number by the type of account you have. For example, Sitebeam Premium comes with 10 Users (1 administrator and 9 builders) whereas Sitebeam Light comes with 1 User (the administrator).
Viewers can be assigned permissions to see reports, but cannot add websites or run reports. There is no limit to how many viewers can be added to an account.