solved the problem with enrolling students via users add form

Instructors can now enrol students via users add form by entering
the students/tutors usernames into the username field. If the
username is found in the system a link would be given to the user
to use if they would like to enrol the student/tutor. Note: this
can only be done using the users add form url with a course id
(eg. via course home page). If an admin use the users add from the
users index page, a duplicate username will only give an error.