To add a user to your site simply click on the Participants menu and then on "Enrol users."
After that, search for users in the "Select users" pull-down menu. If the user will need to have a role in the course other than as a student (for example, if you are adding another instructor) change the role in the "Assign role" drop-down menu.
Finally, click on the Enroll Selected Users and Cohorts button: