Should your students forget to enter the invitation code when they first sign up it is not too late!
Have them go to the upper right menu and select the Groups link. On the Groups page they will see a field in the upper right where they can enter the code. Paste the code there and click Join!
After using the code they will see your group on that Groups page. Beneath the group is a charcoal button to ‘request to join.’ Click that button.
You (the teacher) will be able to accept their request on your own Groups page. Click on Groups from the upper right menu to see the requests.
With those steps, your students will be properly in your group.