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). 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.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. Country of Domicile enter the joint sponsor's country of domicile (most sponsors will enter "US" as the country of their domicile). 6. Proof of U.S. domicile is required for the I864. Learn examples of proof of domicile along with proof of domicile documents required. This ready reference guide helps assessors and others handle the many questions and problems they face in the daily discharge of their duties.