There are two ways that you can enroll users: directly add or allow users to enroll themselves.
Once you are in your Organization, a Leader will see Organization Management on the left side panel. Select Users and Groups, Users.
To directly add:
Select Find Users to Enroll. If you know the correct username, enter username in the Username box and click Submit. If you aren't sure, click Browse. Enter a first name or last name and click Go. Select the user(s) and click Submit.
To allow users to self-enroll:
Select Customization, Enrollment Options.
Select SelfEnrollment, add a start and/or end date if desired. If you wish to limit the enrollment, you can select Require Access Code to Enroll and add a password-type code that you will provide to users. Click Submit.