To add an individual student to your class, you'll first have to create a class to put them in.
Once you do that:
1. Navigate to your Students tab and click on Create Student Accounts in your Teacher Site. You can also scroll down and click on Add a Single Student. 
2. Select "Create a Single Student Account"
3. Fill in the information in the fields.
You’ll then click Save.
Please make sure you choose a unique username as usernames cannot be used more than once.