A: Individuals who can sign the employer's side of an I-9 document are any authorized representative of the company. The DHS does not require the representative to have a specific agreement authorizing them to do so. Your company would be held liable for any violations.
The person who signs a W9 must be a "U.S. person." That means that the signer must be an individual who is a U.S. citizen or a U.S. resident alien. The signer should have the organization's consent to sign a W9 on its behalf, but consent can be oral or implied. There is no requirement that a company create a written authorization to sign a W9.