Sharing notes via tags by adding email does not prompt inviter/invitee without Simplenote

Inviting a new user to view/edit a note does not prompt the invitee or the inviter in any way.

What I expected:

1. I want to share a note with a friend who does not have Simplenote.
2. I add their email as a tag to the note.
3. a. My invited friend will receive an email inviting them to join Simplenote and view/edit the note.
3. b. Adding an email that does not have an active Simplenote account will notify me that my friend must have Simplenote installed to view it.
4. My friend will join Simplenote later and will be able to see the note that I shared.

What happens:
1. I want to share a note with a friend who does not have Simplenote.
2. I add their email as a tag to the note.
3. a. No information is provided to my friend about being invited to share/edit my note.
3. b. No information is provided to me that the invitation has not gone through.
4. I contacted my friend and asked him to sign up to Simplenote with their invited email address, but he does not see the note even after signing up.

Suggestion:
3. a. Send an email to the invitee that a note has been shared and they need to sign up to view/edit it.
3. b. Prompt the inviter that the sharing did not go through and will only work after my friend has signed up for Simplenote.

The issue names no files, tests, or entry points. Start by tracing the macOS app's email-tag sharing flow and how invitations are handled for users without Simplenote accounts; done means the invitee is notified, the inviter receives clear feedback, and the shared note becomes available after signup.

Written by the indexing model from the issue text.