You fill out I-864. read the instructions before doing it. You husband fills out I-864A. The joint sponsor fills out his/her own I864.
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If the cosponsor qualifies financially for the Affidavit Of Support based on last year's taxes alone, then all that co-sponsor needs to do is fill out the "EZ" version of the I-864, with his wife (as a "household member of his) filling out the "A" Supplement.
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Great responses by the best two immigration attorneys in the US - Mr. Segal and Mr. Behar. This just goes to show you that you NEED an immigration attorney for your case. It is not a DIY project. Attorneys are quite reasonable in their fees and are lifesavers.
The information provided in this answer does not create an attorney-client relationship and is not considered to be legal advice.
I strongly recommend seeking the assistance of an immigration attorney to assist you in filing these documents.
BRImmigrationLaw.com: This is not legal advice as many facts are still unknown, and an attorney-client relationship has not been created.