Have you tried creating a Homegroup on the Windows 7 machine, then join the Windows 8 machine to the new Homegroup?
I can't do that either. The existing Homegroup is detected on my network so I cannot create a new one and I don't know the password to join the existing one.
There appears to be a Homegroup trouble shooting app built into Windows 8;
Looking, but not finding anything helpful yet. Most of the problems involve just leaving the Homegroup...which I can't do.