From Longman Business Dictionary
signatorysig·na·to·ry /ˈsɪgnətəri-tɔːri/ noun (plural signatories) [countableC] formalLAW one of the people or countries that sign an official agreementWe will accept a copy invoice which bears a company stamp and the signature of an authorised signatory.
signatory to/ofMost Western countries are signatories to this agreement.
Experts from 118 signatory countries met in Geneva.