If a user has been manually added to a course but has not yet accepted the course invitation after a period of time, you can resend the course invitation. You must allow at least 24 hours before resending a course invitation.
Note: Users are not sent a course invitation until the course participation start date. (The start date is commonly the term date, unless the term is being overridden by a specific course or section participation date in Course Settings.) Additionally, courses must be published for students to receive course invitations.