There are basically two ways to add students to groups/classes:
1. They need to add a class name at the time they register for Annotation Studio, i.e. entering your group/class name into the "Class" field. It´s important to hit return after entering the class name so that the class name appears in a blue box.
2. Students can add the class under their "My Profile" section. They just need to enter your class under "Classes" and hit return so that the name appears in a blue box.
The documents that you shared with the two groups need to have the same class/group names attached to them. You can add this information either during the document creation process or afterwards by editing the document form.
Hope this helps,