The notes are stored as a note elements inside a page. You could for exemple create a “Notes” page and set it as a System Folder. Once you insert the frontend plugin into one of your site page, you just reference the later System Folder.

When you create a note from the backend, you'll be ask for the title, the content, the read permissions and the write permissions. The title and the content fields are self explanatory. The read permissions and the write permissions must point to existing frontend users. If more than one frontend users is assigned to a note, then this note will be shared across those users.