The problems I see with Homegroups is that certain files build up under peer networking and they need to be deleted, these extra files will make certain machines vanish from the Homegroup or become inaccessible. The other symptoms are that you can't leave or join a Homegroup, this tends to occur most often with wireless machines.
You should be setting up the shares from the C:>Users
folder rather than the C: drive. This is the default setting and it's safer than sharing the entire C: drive.
If there is any type of third party anti virus software installed then it will need to be configured to work with Homegroups or removed from the system using the special uninstall tool.
If it's the peer networking problem just go to the location listed in the picture and delete the files below the black line. It might be required to delete all the files in the peer networking folder but deleting the ones below the line seems to work for me. It will probably show more files to delete than in my picture.
Many other Homegroup problems can be fixed by going through the guide in the link. Ultimate Troubleshooting Guide for Windows 7 HomeGroup Connection Issues - Help Desk Geek