These additional financial sponsors also have to submit an Affidavit of Support, proof of their income, and proof of their legal status in the United States. If you want to sponsor someone for a family-based green card, you need to provide proof of a U.S. domicile in your Affidavit of Support (Form I-864).Each joint sponsor must fill out a separate I864 form. In their form, they will include the names of all immigrants that they will personally be sponsoring. Providing proof of domicile in the United States is critical for most people when submitting Form I864, Affidavit of Support. The purpose of Form I-864: (1) Is to create a legally binding contract between certain IV applicants, their sponsor(s) and the US Government. Find step-by-step tips on filling out USCIS' Affidavit of Support form (the I-864).