An Invitation Code is used by schools that use picture passwords and are not rostered (see What’s a rostered school?).
Invitation Codes are one way to add or transfer students. Teachers generate a code, and students enter it into their devices to associate them with the correct class. Invitiation codes should only be used once per student.
You can see the complete details on how to use Invitation Codes at How do I create classes and add students (non-rostered)?