There's a couple ways for students to join your class. The first way is for classes using Google or Clever Single Sign On.
Each of your classes has a unique ID (the invite code), as well as a url that students can head to to join your class.
On the left side of your screen you should see an invite student button.
Pressing this will provide you with the url your students can go to and join your class.
You can become a student in your own class and see what your students will see when they go to join your class by heading to that URL. After a student heads to that URL they will be prompted to login if they haven't already (if they don't have an account, they will be prompted to create one).
If they're logged in already when they head there (or after they've finished creating an account / logging in) they will see the name of the class they're trying to join as well as a join button.
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.
Students that already have accounts can join your class via the Join Class option under the dropdown in the top right of Creator when they're logged in.
They will then be prompted to input the invite code that you can find on your class info page.