What the message means
When you're trying to mention a user in a note using the @ key on your keyboard, you might encounter this message: "Sorry, no other users can see this. Change this".
This message means no other users have access to the lead, process, or inbox conversation you are typing the note on, and you can't @mention them as a result.
How to change it
To get rid of this message, you can grant other users permission to access the inbox, lead pipeline, or process type where the lead, process, or conversation is located. Once they have access you will be able to mention them in Notes on that lead, process, or conversation.
NOTE: You will need admin level permissions to give other user's access to Pipelines, Process Types and Inboxes, or you can ask an admin to do it for you following these instructions: