How to share one iTunes library with other users on same computer?

I've read several postings regarding this topic but so far I didn't have a clue and it drives me almost mad.

Situation:

User A authorised user B for iTunes library of user A

iTunes of user B is able to show the index of all the music from user A, but there is no possibility to select the playing list of user A in order to copy the music to the iPhone of user A. Only the iTunes library of user A is visible and can be synchronised.

Most of the manuals describe the way new music bought in the iTunes store can be shared. This doesn't interest me, the only thing I want to figure out is how to share my existing iTunes library with hundreds of imported CD's.