You will need to show the joint sponsor is a US citizen or a green card holder.
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A separate I-864 in which the person indicates that s/he is a join sponsor.
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Your husband will need to send a letter explaining that he is unemployed and did not earn sufficient money to be required to file income tax returns.
Your joint sponsor will need to complete and sign an an additional I864 Affidavit of Support, send proof of US citizenship or LPR status, send a copy of his W2 or 1099 for the last year, and proof of current income.