If you've missed a student when creating your class you have different options:
1. You can give the students the class code that you'll find in the list of your classes or at the right side of the class page. They can create their own user account via this link. Here they'll be able to enter the on their English Every to join the class.
2. You can go on the class page and click on the 'Invite students' page. There you can follow the same steps as you did when you invited the whole class: enter the last and first name and the email address of the missing student. Alternatively, you can go on 'Share the class code' to send an automatic email with the code.