The Collaborative International Dictionary of English v.0.48:

Signatory \Sig"na*to*ry\, a. [L. signatorius.]
   1. Relating to a seal; used in sealing. [Obs.] --Bailey.
      [1913 Webster]

   2. Signing; joining or sharing in a signature; as, signatory
      powers.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Signatory \Sig"na*to*ry\, n.; pl. -ries.
   A signer; one who signs or subscribes; as, a conference of
   signatories.
   [1913 Webster]

WordNet (r) 3.0 (2006):

signatory
    n 1: someone who signs and is bound by a document [syn:
         signer, signatory]