First of all, I am no expert, but I had the same requirement and found a way to do it . I dont use MIONET, because of repeated problems with being able to see the MBWE, so I do everything through the SSM interface
Access the Shared Storage Manager on the Mybook ( whatever you called it eg MYSTORAGE )
Create a userid and password for each of your roomies (eg Roomie1,pw1 etc )
Create a share for each roomie, so that only they can read/write to it. (eg Folder1, Folder2 etc )
When they want to access their private share, your roomie should go to MAP NETWORK DRIVE (tools from MY DOCUMENTS)
The folder should be specified as
\ MYSTORAGE \ FOLDER1
They should click on Connect Using a Different user name , and windows will present a dialog - they should enter the userid and password you created for them on the SSN - Roomie1 pw1 etc
Then they can access their own private space.
Windows will only allow you to map to the same disk once - so your roomie cant access the PUBLIC space under one userid and at the same time access their private space with the userid you created. So they may need to disconnect from PUBLIC before accessing their private space.
Works for me, but I had to figure it out for myself - someone more experienced than I may frown upon it.