if you're running multiple windows systems with a server and DONT have centralized authentication, you have a mess.
if you're not running windows systems, then why are you using SMB ? NFS is the native file sharing system for Unix and Linux systems.
It is a bit of an oddball arrangement. We are a scientific research lab within a hospital environment. Most people use Linux or Mac - and those who do use Windows connect to the hospital domain. We don't have any control over that domain at all - I support the researchers and am independent from central IT. When the Windows users connect to the 5.3 box via SMB they use a local username and password on the 5.3 box, not their central domain credentials.
Hmmm, I probably know what the answer will be, but I could always ask the hospital to let me connect it to the domain. Though that could present security risks that I don't want to deal with.