You edit the sharee role notes either through the Edit Person screen of the Principal, or through the Edit Person screen of the sharee. To add a note only to a sharee, click on the shared event fact/item and then click on the "Share" button. This will open the box of "People that share this event". Highlight the person/sharee and click "Edit". You will now see the "Edit Shared Event" screen, below the role is a note field that can be edited. This note will appear after the role sentence for someone sharing a fact/event with the Principal.
Note: If you just click on the note column, or the note button of shared item, in anyone's edit person window, anything you add or edit shows up for the principal and all the sharee's edit person window.