Identities and permissions on a workgroup are on a per-machine basis, but you can log into the machine remotely using the identity on the machine. For example, if you have a network with two machines: Zen and Orac:
jonesf can log onto Orac using their login, but cannot login to Zen as they have no account on it. However,
bloggsj can grant permissions on their share to
Without a domain controller or other centralised identity mechanism you cannot centralise identity on Windows. If you don't want to shell out for Windows Server there are some alternatives available, some of which are free:
Samba. Setting up a PDC in this is somewhat fiddly but is possible. Note that you need 'pro' versions of windows as 'home' versions do not support connecting to a domain.
Open-source authentication mechanism such as pgina or NISGina, although I don't think either of those are actively maintained any more.
Proprietary identity management systems such as Novell eDirectory (Formerly NDS).