By default SharePoint imports the users from the current domain (domain which the SharePoint server is joined to). But if you work for a big corporate you might need to create more domains in your forest. Most practices will answer that with an Extranet, I would answer with: Import the entire forest
Extranet's are excellent practices, however they need provisioning, planning, and proper change management. I would encourage this approach if I want to share my information with partners and clients, but when it comes to internal parties I don't.
To import the entire forest to your SharePoint profile store follow the following steps:
- Browse the SSP site using this URL: /ssp/admin">/ssp/admin">http://<portalname>/ssp/admin, you will need a farm administrator user account to be able to login. Then click on User Profiles and Properties under the User Profiles and My Sites section.
- Click on Configure Profile Import under Profile and Import Settings.
- Choose the Entire Forest option under the Source section and click OK.
- Click on Start Full Import to get the users from other domains.
Now all the users who are members of other domains in your AD forest have access to your portal.