A signatory is someone who signs a document and is subject to it. The co-signer for a loan is one type of signatory.
A signatory is someone who signs a contract, therefore creating a legal obligation. There could be several signatories for a specific contract. Over time, this word has often been used for a person or country who signs a peace treaty. If the treaty is broken, the signatory will be blamed. You could be a signatory for a marriage, mortgage, adoption, lawsuit, or employment contract.