Windows 7: Proper HomeGroup Behavior

I have 5 machines on a home network, all running windows 7 (either x86 or x64). They are all part of the common homegroup. Each machine has 2 or 3 users. However, the homegroup dropdown in Windows Explorer shows just one user per machine, and it is always the same user:

Frank (Machine1)
Frank (Machine2)
Frank (Machine3)

Why does it not show me one or more of the other users, perhaps like this:

Frank (Machine1)
Amy (Machine1)
Frank (Machine2)
Amy (Machine2)

The undesireable behavior, where everyone only sees "Frank," occurs regardless of who is currently logged into a given machie. Why do the other users not show? I've taken care to ensure that other users are sharing "documents," through the homegroup but that changes nothing.

But the story gets worse.

In Windows Explorer, I next try to use "Network" drop-down to access the files of another machine/user. I try these scenarios:

From Machine1/User "Amy" I can see Machine2/User/Amy. Good! I try to copy files to that location, and I get a permission denied.

Next scenario:

From Machine2/User "Amy" I can't even see Machine1/User/Amy. Huh? How does that make any sense?