Yes. Anyone you share with has the option of creating an account using their own email and password. They do this by clicking the link at the top of the page while accessing what has been shared with them online. They can then download the desktop app and have access to the pogoplug as a drive. The main account holder controls what each person can access based on what they choose to share with them.

My brother and I share two PGs with one main account. We registered our own PGs with our email addresses and the same password. Then he added my email account to his PG and and I confirmed the password. Now we have two email accounts sharing two PGs with the same password. All four drives (2 & 2) show up for both of us.
That, my friends, is our cloud.

Yes. If we go to the "Settings->General Setting" we have two email addresses listed under "Your email address" for the single screen name.
When I did it I had created my own screen name but it was removed when I authorized 'his' account. So I guess that it is really one account that accepts either email address.

I assume what you want to do is have the LAN owner of the PogoPlug register with their name and address then add the other persons email address and tell them the password.