Secure Networking with Windows

When you set up a share in Windows, you can specify permissions for that share under the Security tab. First thing to do is remove "Everybody" group from permissions and then just add users who need access to it. Give those users Read and only Read, unless its a dump folder for them. Short of putting in GPOs, this is about as secure as you can get. If there's too many users to manually manage, create a group and stuff them all in there, then add that group to the permissions with whatever permissions they need.

Jam it back in, in the dark.

Set up users for remote computers? If you remove the Everybody group, set Public Sharing to off, and turn Password Protected sharing on, the remote computer should ask for a user/pass when the user requests the share, in which the user would enter a user/pass for login. If you're looking to have the computer login based on the individual computer's login credentials, then you're starting to get into policy objects and things get "fun" :P

Workgroup only matters for the whole "computers near you" aspect. When you browse for computers, computers in the same workgroup will be listed immediately. You can still find any computer on any workgroup within your network by browsing the entire network, or manually going to it (\\servername\).

EDIT: I should clarify something real quick. Generally, if you set up a user/pass on the fileserver that matches that of a user's windows login, they can usually just access the share, because Windows file sharing will attempt the user/pass associated with the Windows login first. However, this isn't always the case, as Windows file sharing is a magical beast that only works when it wants to

There's nowhere I can't reach.