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. The I864 Affidavit of Support is a signed document that a sponsor signs in order to accept full financial responsibility for the sponsored immigrant.This video will help you to fill out I-864 For Joint Sponsor. If your income is insufficient to sponsor an intending immigrant on Form I864, you may enlist the additional support of a joint sponsor. This tip sheet will help you complete Form I864, Affidavit of Support Under Section 864A of the INA. Detailed Instructions for Affidavit. You, the sponsor (petitioner), and the joint sponsor must complete Form I-864. See tips for completing Form I-864. Note that I864A is a different form. They must also complete a Form I864A, Contract between Sponsor and Household Member.