Network User authentication

I know that PostGres can do about half of this, what I want to know is
whether the other half is possible.

We are setting up a postgres database on the company network and want to
know who makes changes
and when. I know that through the pg_hba.conf I can make sure that only
people on the network are allowed
access, and can assign them as users or groups.

What I would really like to do is recognise each user as they access the
database. Ideally I would like to
do this without having to setup the users in postgres in advance.

Is this possible???

Dan Kelly