A signatory is a person, a company, or a country that puts their name on an official agreement and so promises to follow it. News stories use it a lot for countries: the signatories of a treaty are the countries whose names are on that treaty. It is a formal, legal-and-newspaper word. In ordinary talk you would just say “the people who signed.” Though actually, it also works as an adjective — signatory states, signatory parties — and that is the same idea, not a second meaning: the ones who signed. It names the signer, not the pen and not the mark on the page.