Sorry you’re having trouble!
I have inspected your annotations on A Girl. While it’s hard to be certain, it appears that you and firstname.lastname@example.org are both doing everything right.
I’ve joined that group, and reloaded the page, and I can now see your annotations. Bear in mind that if at any point you change your group memberships (on your user profile), that that doesn’t change which groups your annotations belong to.
Try opening and saving your existing annotations, and ask your classmate to do the same, and let me know if that doesn’t rectify the issue.
Also, even once you’ve done both of those things, you may have to refresh the document page to get the annotations reloaded with your new groups included.