Use this form to show they have adequate means of financial support and are not likely to rely on the US government for financial support. 30 The joint sponsor must sign a separate Affidavit of Support, Form I-864.You, the sponsor (petitioner), and the joint sponsor must complete Form I-864. See tips for completing Form I-864. The "Affidavit of Support" is a signed document to accept financial responsibility for a family member who is seeking a green card. In today's video we are going to be filling out form i 864 the affidavit of support form for the joint sponsor. An Affidavit of Support (Form I-864) is a document that says someone agrees to financially sponsor an immigrant who has applied for a green card. There are two different forms that are used for the Affidavit of Support: the I-134 and the I-864. The person who is petitioning on behalf of the immigrant visitor fills out Form I-134. They must also complete a Form I864A, Contract between Sponsor and Household Member.