A guarantor agreement is an agreement of a third party to provide assurance of payment in the event the party involved in the transaction defaults. Edit, sign, and share Guaranty Attachment to Lease for Guarantor or Cosigner - Illinois online.Sample guarantee agreement for individuals and businesses at 360 Legal Forms. Create, print, and share free with quick sign up. This legal template is designed for landlords and tenants in Illinois who require a personal guaranty for a residential lease. A form of guarantee agreement is an agreement where a guarantor promises to fulfill the financial obligation of a borrower if they fail to pay.