I have two google accounts, call them A and B. A shared a folder, call it A-Drive, with B. B should be able to do an "Add to My Drive" -- but that action is not available. How can this be fixed?
You can add just a shortcut (from 3/2020). There is a workaround: you can use the keyboard shortcut Shift+Z and old the feature appears. You can add the shared folder to your Drive (works also in Google Sync).
More info here.
The problem turned out to be caused by having a situation where A and B had both been logged in in the same browser at some point in recent memory. Here's the procedure I finally found to solve this problem:
Use two different browsers for this work, e.g. Opera and Chrome.
In Browser A, log in as B, then log out; now log in as A.
In Browser B, log in as A, then log out; now log in as B.
Unshare the A-Drive folder and save. Now share it again.
Now you can go to Browser B and do "Add to My Drive" for the A-Drive folder.