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?
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.