Countersign

[Sarah]

To sign on the opposite side of (an instrument or writing);hence, to sign in addition to the signature of a principal orsuperior, in order to attest the authenticity of a writing.

[Jessica]

to write your name on a document that already has the signature (= name written) of another person, especially in order to show that you are certain that the first person is who they say they are:

[Jason Yan]

Add a signature to (a document already signed by another person)