Please also see https://www.odoo.com/forum/help-1/error-assigned-user-has-no-access-to-the-document-and-is-not-able-to-handle-this-activity-201375 to understand how to modify the record rule so that when you assign a User to an Activity on a Private Note, they will automatically be added as a follower and will see the note. 

Hi Jerome,

With Notes (module) there's one way to link attachements with people: it's to add each follower.
I think it's more convenient to use Document (module), there you can create a specific channel "sales team" eg/.

I hope the answer was helpful to you.