You may need to reload/refresh the page for the student to appear in your list of students.

If a student does not appear, it could also be because the student used the wrong code (although there is a check before).

If you use Google Classroom or Clever and have already imported your students, you can always "resync" by clicking on 'Invite students'. Useful if a student has subsequently joined one of your classes on either Google Classroom or Clever.