This error message has been kicking my ass today. It turns out I had two problems:

1. The homegroup was created by a computer that is no longer on my network. Solution: Leave it form all machines, and create a new homegroup.
2. IPv6 needs to be enabled. For some reason, homegroups only work over IPv6. Solution: Enable IPv6, and welcome to the future.

Edit: I spoke too soon. Still cannot make two computers talk to each other. I am going to have to revert to good old sneakernet and a USB drive.