My sister wanted to do this, so here's what I set up for her:
If you're just starting out, move the "iTunes" folder in "~/ Music" to "Users/ Shared".
In your user account, create an "iTunes" folder shortcut in "~/ Music" that points to the one in "Users/ Shared". Do the same thing for all additional user accounts.
If you already both have separate libraries and you want to combine the music in each of your accounts, you'll have to choose one library as the one to move to "Users/ Shared", and then copy the others' files into what's now the source library.
If it all goes wrong when updating someday and you lose your music (though you shouldn't), don't blame me. She wanted a quick and easy fix to this problem, so that's what I did. And it seems to have worked well so far. But I'll probably get a hundred thousand follow-up posts telling me what a dumb idea it is. The only thing I see as a potential hang-up is if you're both logged into your accounts with iTunes running, and one of you is adding files to iTunes.
You can also share iTunes libraries using that function in iTunes. It's safer, I guess, but it requires both of you to be logged in and have iTunes launched.