There are three possible reasons why a student may not receive an email to join your class.
1) The student already has a Khan Academy account. In these cases, the student will receive a notification which they can see by clicking the bell at the top menu. They can then choose to accept your invitation.
2) The email ended up in their spam folder. Have your students check the folder where spam would go and see if the invitation is there.
3) The student is using an email that has a restricted email policy. Many schools restrict email for students to email from approved domains.
If none of these solve the problem, they can create their own accounts and then add you as a coach.