signatory

The authorities disagree. They quickly detained the chief organiser, Liu Xiaobo, a veteran Beijing activist, and threatened or questioned dozens of other signatories. Chinese internet-service providers removed postings about the document. A blog-hosting service, Bullog, home to several personal sites supportive of the charter, was shut down.

Definition

signatory
noun [C]
a person, organization or country which has signed an agreement:
Most western European nations are signatories to/of the North Atlantic Treaty Organization.