It is a big PITA to do this without tying your Home Sharing Apple ID to your real iTunes account that has a real source of funding attached to it. Basically you follow this article:
http://support.apple.com/kb/ht2534
Having this be an official Apple support article makes me wonder if Apple got a whole lot of complaints from people that don't want to connect a credit card to an iTunes account so Apple created this back door as a way to do that. Not sure why they can't just allow an Apple ID to be created and used with iTunes for Home Sharing without requiring the user to supply payment information. Seems like maybe the Apple ID and iTunes Store and Home Sharing teams don't talk to each other. Oh right, it's Apple - they don't talk to each other.
I love Apple stuff - been using it as both a consumer and a developer for many years, but this whole recent mess with abandoning the Apple TV Gen 1 and then coming up with Apple TV Gen 2 and then only supporting Home Sharing with all the crap you have to go through to just do basic sharing and the inability of the Apple TV to do more than one Home Sharing account is really starting to wear on me.