Students that already have accounts can join your class via the Join Class option under the dropdown in the top right of Creator once they're logged in.
They will then be prompted to input the invite code that you can find on your class info page.
After your students type in the invite code for your class, they will see the name of the class they're trying to join and a Join Class button will appear.
All they need to do at this point is click the Join Class button to join your class. Afterwards you should see them in your roster, and they should now be able to see any lessons you've assigned to the class they've joined.