Providing proof of domicile in the United States is critical for most people when submitting Form I864, Affidavit of Support. Completing and signing Form I-864 makes you the sponsor.Line-by-line guidance for properly filling out USCIS's I-864 Affidavit of Support to sponsor an immigrating family member. 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. Affidavit of Support is a legal contract between the petitioner (a person sponsoring a family member for permanent residency) and the US government.