Scenario-1 - All users are in the current domain.
1. Go to User Profile and Properties under Shared Services Administration.
2. Click on configure profile import link. Select current domain as the profile datasource.
3. Make sure default access account is specified and is valid.
4. Run full import or incremental report.
Scenario-2 - Users are scattered into multiple domains or other data sources.
1. Go to User Profile and Properties under shared services administration.
2. Click on Import New Connection. Add a new connection for each of the data sources where you users are located. Delete any unwanted connections that might already be there.
3. Configure the import as mentioned above and make sure that Custom source is selected.
4. Run the full import or incremental import.