If you've accidentally removed a student from your class, the easiest way to add them back is to use the 'add a student' function. 

Step 1: Go to My Classes and click into the class with the student you want to add back. Then click on My Students.

Step 2: Click 'Add students'. Use the option 'Add an Individual Student'. Click 'Invite student'. They will automatically be re-added to your class.

Note: Make sure you get the email address correct, otherwise you might invite a completely different student! Or no student at all!