user-123 is a member of the Central IT organization on the Satellite, the following command on any system would register that system to the Central IT organization on your Satellite:
rhnreg_ks --username=user-123 --password=foobaz
--orgid (for Red Hat Enterprise Linux 4 and 5) and --orgpassword (in RHEL 4) parameters in rhnreg_ks are not related to Satellite registration or RHN Satellite's multiple organizations support.