Share permissions

I work for a company running Windows server 2003 on about
20 XP computers on native mode. We use one domain under
the Active Directory. Each computer can access data from
the network, see all other computers on the network and
open files on other computers but when sharing a file,
clicking Permissions, and Add, the only location
available is the individual computer and its local users.
our server/domain controller is the only computer that
has the entire domain under location (obviously). What
can I do to fix this problem?

Thank You

Pablo Calderon