Shared Notes can be created and seen by any user who is a member of atleast one Shared Label added on the email conversation. You can do a @username to direct a Shared Note at any user.
To add and see Shared Notes on email conversations you will need the Hiver browser extension – you can download it here. Once you’ve done that, refresh your Gmail, and you’d see a box to enter notes right next to every email conversation. This is how notes look:
Tip : You can address multiple users in one Shared Note by writing @username1, @username2..etc in the note. All the users mentioned will get the notification.