How to Join an Existing 2003 Domain to an Existing 2003 Forest?

I have 8 remote facilities running on their own 2003 Domains. I need to join each of them as trees in the forest hosted by our Corporate Domain Controller. Each user should be able to login to the Corporate Domain from any remote facility while retaining rights to their local shared resources.

I am seeing lots of help that refers to upgrading from 2000 or migrating from NT, but nothing that talks about joining 2003 Domains to an existing 2003 Forest. It has to be easier than what is described for NT!

Any help would be appreciated.

Pete