what I found in windows 7 that I've seen nobody document: even though you may have a folder or three marked as shared via homegroup, that will not suffice for this purpose. I have a lot of things shared, but that's between local windows systems on the net. what it needed was to go to the advanced sharing settings for any one folder and share with "Everyone" [its on the pulldown menu]. do that, and it will go.
the other thing is that if you're on a AP with dynamic addressing, the machines will get differing IP's as they come online and go offline - this is why folks recommend setting static IP. to me, it ain't that big of a deal - once you get the sharing permissions working once, its quick then to simply scan for your fave machine and latch it again.