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 with their Google or Clever account. Doing so will create a GameSalad account and link it to their Google or Clever account (if they don't already have a GameSalad account).
If they're logged in already when they head there (or after they've finished logging in / creating an account) they will see the name of the class they're trying to join as well as a Join Class 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.