Some progress but still struggling to master W7's HomeGroup

As I posted yesterday, this potentially great feature of Windows 7 is giving me some headaches. However, I made just a little progress although there's still quite a way to go.
However, using just one desktop (Dell Dimension 9200) and one netbook (wireless internet), by turning off the router firewall, no improvement was made over what I had reported yesterday (netbook not able to join the desktop's Homegroup).
But, after turning the router FW on again but then turning off the W7 firewall, the netbook immediately showed the desktop as a Homegroup computer.
What's even stranger is that when I then turned the FW back on, I didn't lose the Homegroup connection.......even after a reboot.
Now, with these two computers still running, I fired up a further desktop (Dell Dimension E520) running Windows 7 Ultimate.
Even though this showed it was joined to a homegroup, the documents folder showed no homegroup computers.
To overcome this problem, I had to leave the homegroup which was a different one to that connecting the netbook and the Dimension 9200.
Thereafter, the Dimension 9200 Homegroup showed up as being joinable. So, I joined using the same password as for the other two computers.
However, things still weren't quite right. Here's what other Homegroup computers all three machine were seeing:

Dimension 9200 only saw the Dimension E520
Dimension E520 only saw the Dimension 9200
Netbook only saw the Dimension 9200

Need to investigate this quite a bit more.