In the "Additional Options" menu in the eReader, you will see a header called "Collaboration," which allows you to share your notes in the eReader with others and allows them to share their notes with you.
The first tab under "Note Sharing" will allow you to share your notes with others. A unique code is generated, per eBook, that you can provide others with to gain access to your notes. If at any time you choose to stop sharing, you can toggle off "Share my notes." If you want to remove sharing access, you can generate a new code, which will invalidate the previous code and remove access for everyone on the old code. Those with whom you share your notes will only be able to read your notes. They will not be able to read your eBook.
The second tab, "View and Subscribe" allows you to subscribe to someone else's notes. Similar to how you share your code, another person can share their notes with you. They will provide you with their code (unique per book), and once you enter the code and click "Subscribe," you will see the sharer's notes in your book. Additionally, you can toggle on/off seeing someone else's notes or remove their notes entirely.
Once someone has shared their notes with you, they will appear in the reader. Your notes will appear as "My Notes" and the sharer's notes will appear as "Shared Notes."