The word "signatory" refers to a person, organization, or country that has signed an agreement or a treaty. It means they have given their formal approval and agreed to be bound by the terms and conditions stated in the document. Full definition
The notable collection of signatories includes four Turner Prize winners — Simon Starling, Mark Leckey, Jeremy Deller and Elizabeth Price — as well as Sir Anish Kapoor and -LSB-...] (artlyst.com)
Other signatories of the letter include former Bishop of Rochester the Rt Rev Michael Nazir - Ali and CEO of Christian Concern, Andrea Williams. (premierchristianity.com)
Update, 4:15 p.m.: This post has been updated to note that some people listed as signatories on the letter have said that they did not sign it. (artnews.com)