A cosigner for an apartment takes on the financial responsibility for a lease. Learn how having a cosigner can help someone get approved for an apartment.Learn the essential steps on how to add a cosigner to a lease to strengthen your rental agreement. A cosigner is added to a lease before it's signed, and they guarantee to cover a tenant's missed rent payments. Use our Lease CoSigner Agreement addendum to extend the legal terms of a tenant's lease to a cosigner. A cosigner is someone who signs the lease along with the main tenant of the rental property. Com or ask your property manager about Leap. Edit, sign, and share Landlord Tenant Lease Co-Signer Agreement - Nevada online. We only accept Nevada residents as cosigners. We will act as your co-signer to help you qualify for an apartment or rental home.