A maximum of five computer can use Home Sharing at one time. If this limit has been reached and you want to add a new Home Share, turn off Home Sharing on one or more machines using the Advanced menu in iTunes.
I think the OP might be talking about the old iTunes sharing feature (that lets you connect to someone else's library and play their content, but not sync it) as opposed to the new home sharing feature that lets you synch content?
Sharing your iTunes library across computers
If your computer is connected to other computers over a local network, you can share items in your library and playlists with up to five of those computers (in the same subnet).