Only meeting attendees (ones on the calendar invite) will be able to see the contents of the notes. These faces or icons will appear at the top of the note.
If you write anything in the Private Notes section on the far right hand side, they will only be visible to you. And if you share your screen you can hide them from view by clicking on the "eye" icon.
If you are meeting with people outside of your organization (external attendees), they will NOT be able to view/edit the notes in Fellow.
However if you would like them to see/have a copy of the notes, then you can send out the agenda and notes via email or Slack.
(But they will not be able to "Open in Fellow")