Joint Sponsor

A person who accepts legal responsibility for supporting an immigrant with an I-864 Affidavit of Support along with the sponsor.  The joint sponsor must be at least 18 years of age, an American citizen or lawful permanent resident and have a domicile in the United States.  The joint sponsor and his/her household must have the 125 percent income requirement by itself for the immigrant that he/she sponsors.

Source: U.S. Department of State.