This word is acceptable for play in the US & UK dictionaries that are being used in the following games:

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n. A cosignatory

n. A person who accepts joint responsibility for the debt of another

n. one of two or more signers of the same document (as a treaty or declaration)

n. a signer in addition to the principal signer (to verify the authenticity of the principal signature or to provide surety)

"Normally, a cosigner is required, and a one or two month rental deposit can be applied to the last month's rent."

signer    signatory