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 icon (three heads). 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!
If they are doing this within 30 minutes of registration, they are good to go. It a little more time has passed there are a couple more steps. 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 students page. Click the Students menu and at the top you’ll see the requests and will be able to accept them.
With those steps your students will be properly in your group.