You can set up your site to automatically assign courses when a learner registers into your site. This can be done on the Site Building page in Admin Settings.
Under Registration Courses, select the courses you'd like to automatically assign, then click on Save Changes.
See also the Site Building page.
Reporting Groups course assignment
Courses can be assigned through Reporting Groups. Once a Reporting Group has been created, courses can automatically be assigned when learners are added to the group.
To learn more, go to the Setting up course assignments in Reporting Groups.