access by password to some unit

Hi everyone,

I have 6 PC in a WORKGROUP network via a Dlink switch. 3 of them need access to all other without any restriction, but the other 3 can access to the first 3 only with a password and with the ability to disconnect from them at any time. The purpose is to share some folders (c:/share or c:/users/share) and files. Here are the details of each PC:

PC1: Win 7 Pro x64 - The big boss which can access everywhere with no restriction
PC2: Win 7 Home x64 - can access everywhere with no restriction
PC3: Win 7 Pro x64 - can access everywhere with no restriction
PC4: Win 7 Pro x64 - access to PC1, 2 et 3 with password only and the ability to disconnect
PC5: Win 7 Pro x64 - access to PC1, 2 et 3 with password only and the ability to disconnect
PC6: Win 7 Pro x64 - access to PC1, 2 et 3 with password only and the ability to disconnect

Now every PC can connect to other ones but I don't know how to set a password (or any restriction) for PC4, 5 and 6 to access to PC1, 2 and 3. I hope you understand my question (I speak better french than English ^^). Thanks for any help, link, advise... :)