Drag the whole iTunes folder from the Music folder of the one user to that of the other, putting it in the same relative position in the drive heirarchy. You may have to authenticate to do this. You will likely also have to set permissions on the itunes folder and its contents. Read the part about changing permissions on the iTunes folder in: https://discussions.apple.com/message/11583914
What are the iTunes library files? - http://support.apple.com/kb/HT1660
More on iTunes library files and what they do - http://en.wikipedia.org/wiki/ITunes#Media_management
What are all those iTunes files? - http://www.macworld.com/article/139974/2009/04/itunes_files.html
iTunes 9 [and later]: Understanding iTunes Media Organization - http://support.apple.com/kb/ht3847
Image of iTunes folder structure Dec. 2011 - https://discussions.apple.com/message/16923545
Image of folder structure and explanation of different iTunes versions (turingtest2 post) - https://discussions.apple.com/message/13025536 and https://discussions.apple.com/message/17457605
New obstacle: I downloaded an mp3 from the internet, then tried to copy it to iTunes. I got the message "Attempting to copy to the disk Macintosh HD failed. You do not have enough access privileges".
I checked the permissions in the Music folder, the iTunes folder, and the iTunes Music folder. All include my current user as having Read & Write privileges.
Any ideas as to what to try next?