As of this morning, it seems that the Add Additional Artist feature is broken. I'm able to get as far as this dialog box where you search for an artist to add, but after I try searching for any artist, the results are always blank.
This issue also seems to extend to the "Transfer" option under the Remove Song dialog box (which brings up an identical search box for an artist).
This issue also seems to extend to the "Transfer" option under the Remove Song dialog box (which brings up an identical search box for an artist).