samba - May 2001 - where do the users come from?

I'v upgraded to 2.2.0 and now I find that new users added to smbpasswd
are able to logon to my domain.  Very cool.

Under win9x when I'm setting up a share with user-level access control I
get a list of users from my domain, but it's not complete.  Where does
that list come from?  Where do the two groups, Domain Admins and Domain
Users come from?  I'm confused and tired.