Sharing itunes music between two computers
The feature you're looking for is now called Home Sharing. It's essentially the same as the original sharing feature, but it ties into your Apple ID to ensure that you can only share between your own computers.

In iTunes, just go to File > Home Sharing > Turn On Home Sharing and enter your Apple ID and password. Repeat the same steps on any computers you want to share with (up to 5) using the same Apple ID. Once you do so, all enabled computers will "see" each other and let you stream or copy music between them.