What you're asking cannot be covered on this forum. I have a similar setup at home, but I am not sure if you can set up multiple "workgroups" (you'd have to set up an NT domain, actually, to process logins on the Linux server). To have multiple NT domains, you'd probably have to have separate Linux servers (one per domain), but I may be wrong.
The best way to go about this is to download the SAMBA HOWTO collection (I used this same document to set up my network) which can be downloaded in
HTML and
PDF format from
here.
Just to give you a hint, you'll have to set up a PDC server on your Linux box using SAMBA, and make your Windoze clients join this NT domain.
Good luck!