Co-signers can be helpful since they can fill vacancies more quickly, which is beneficial, especially when the rental market is competitive. Use our Lease CoSigner Agreement addendum to extend the legal terms of a tenant's lease to a cosigner.Learn the essential steps on how to add a cosigner to a lease to strengthen your rental agreement. A cosigner is a person who signs a lease together with the renter and, as a consequence, makes a legal promise to pay the rent if the renter fails to do so. A cosigner is someone who signs the lease along with the main tenant of the rental property. Co-signers must also pay an application fee and give the same consent as other applicants. Signer Agreement is a legally binding document that outlines the responsibilities and obligations of a cosigner on a Lease Agreement. All potential cosigners should fill out a rental application, provide proof of income, and agree to submit a credit and background check if your state allows. Co-Signer and Rental Guarantee Agreement. " Having a cosigner is a great way to get your foot in the rental door.