The XBox 360 is a gaming computer, and more. As a computer, it's perfectly capable of sharing files, music, movies, what have you, with any computer on the LAN.
Since the XBox 360 is built on the Windows Media Center Edition platform, (KB887212): it can't join a domain. You'll do better having your computers in a workgroup.
You'll need either Windows Media Connect or Windows Media Center Extender. You may find additional information in:
- Microsoft: Your Entertainment Anywhere.
- UGo.com: XBox 360 Launch Guide.
- XBox.com: XBox 360 Media Connectivity.