Sharepoint Portal Server - Help required

I have been asked to setup a sharepoint intranet site serving over 1000 users.
The users are spread across 16 sites and are on multiple domains using multiple
exchange servers. We are using Sharepoint Portal Server 2003 and I have wss 2
installed. I am new to sharepoint and am not sure what more information you will
require to advise, please post for further info. 

 I need some advice on how I go about setting up the sharepoint portal server
and how to add users across multiple domains/exchange servers. Is this possible?
Will I need to get a list of users from each site an import them manually or can
this be done from within the setup? 

 If anyone has any idea's or know where I can get the answers from, please let
me know asap. 

 Thanks