How to trust a key only for users in one domain

[Chris Covington]

What would the command line be in GnuPG to sign a key and specify a domain, so that if that signed key signs another key in the specified domain, the other key would be listed as valid?

For instance if I want to sign the gnupgAdmin at example.org public key so that any key that the gnupgAdmin signs is listed as valid, if the key is for a user in the example.org domain.  This is typically referred to as a "Trusted Introducer" signature and is listed in RFC 4880 section 5.2.3.14.

Thanks,
Chris