Windows 7 Homegroup does not play nice with bridged adapters. If you are bridging your wired NIC to a wireless NIC (in our case, for the Xbox to talk to our wireless network), you can start a Homegroup and others will be able to see it but they will encounter errors trying to join it. Likewise, if someone else has created the Homegroup, you will see it but not be able to join it from the bridged PC. Taking both adapters off of the bridge and then deleting the Network Bridge adapter resolved the issue for me.
That said, Homegroup is SIMPLE and it WORKS. If we want to watch a show on our office computers but the file is downstairs on the TV pc, we can just browse to the file in our Homegroup and watch it. The people who say it's just as easy to set up SMB shares haven't gone through the painful process of walking Mom & Pop through sharing each folder/setting permissions/creating user accounts/opening firewall ports.