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). On Form I864, petitioners must prove that they have the ability to financially support the visa applicant(s) if necessary.Completing and signing Form I-864 makes you the sponsor. Completing immigration forms can be a complicated process. Form I-864, Affidavit of Support, often generates questions and requests for assistance.