Multiple Companies , 1 Exch. 2000 server

We provide services, including email access to two companies besides our own. We are in the process of migrating from Exchange 5.5 to Exchange 2000. We would like Company A employees to see Company A distribution lists by default (it’s OK if they can navigate to others, but the default view should be their own copmpany). We are having trouble setting up Exchange 2000 to do this – any pointers would be greatly appreciated.