Notes
If you have invited a person to share a note how do they join the note? They see the note but they’re not able to edit.
iPhone 11 Pro
Just to confirm you want to add someone to collaborate with on a note
This is an awesome feature that Apple offers.
Please confirm first of all that you are in the iCloud sector of notes app
This is a good resource on sharing notes : https://support.apple.com/en-us/HT206987
Once you open the note, you tap on the more button (...) then tap share note, then you may tap on sharing options and enable or disallow permissions for the note.
Hope this helps ;)