Having a student join a teacher’s group is a two-step process. It is very common for students to enter the invitation code, be in the teacher's group, but still not be properly owned by their teacher’s account. When this happens, students will not appear in their teacher’s ‘Students’ list and they will not have their teacher’s paid features.
In order to become properly owned, after entering the invitation code students should navigate to their group page. Their group page can be found by going to the upper right menu and selecting the groups icon (three heads).
Once the students have located their group page, they will see their group’s name. Underneath that will be a charcoal button that says ‘Request to Join.’ Click this button. If they do not see their teacher's group or this button, it is because they have not yet entered the group invitation code. They can do that in the field on the right.
After the student requests, the teacher can then approve the student’s request from their Students page. Simply click the ‘Students’ button at the top of the page and the requests will appear at the top.