This video will help you to fill out I-864 For Joint Sponsor. The affidavit is a long-term contract.If you have a joint sponsor, they must also complete Form I864. The affidavit of support is essentially a promise someone makes to take care of an immigrant until they become a citizen. Step 4: Complete Affidavit of Support​​ The person who signs the Affidavit of Support is also called the "sponsor. The Affidavit of Support is a Contract between Sponsor and Household Member. In today's video we are going to be filling out form i 864 the affidavit of support form for the joint sponsor. If you are unsure which form you need, please call the Department of Planning and Development at 703-324-1290, TTY 711. And of course, if your sponsor does not make enough money on your Affidavit of Support, you're going to need a co-sponsor.