You sign an additional one. From the same link as above :
What happens if the petitioner doesn't have enough income?
Petitioners who cannot meet this level have two choices: 1) find a “joint sponsor” who will agree to also financially support the visa applicant, or 2) use the income of a household member to meet the Poverty Guidelines. These additional financial sponsors also have to submit an Affidavit of Support, proof of their income, and proof of their legal status in the United States. Remember: Even if a petitioner finds a joint sponsor or uses the income of a household member to financially sponsor a visa applicant, that petitioner must still submit an Affidavit of Support.