Yes, you may use a Joint Sponsor. The USCIS permits you to use a Joint Sponsor. The Joint Sponsor is a third party and must be either a US Citizen or a Green Card holder. The Joint Sponsor has to also file Affidavit of Support and states that he, as the Joint Sponsor he will be financially responsible for the immigrant and that as the Joint Sponsor he will financially support the immigrant once he gets his green card and becomes a lawful permanent resident of the United States, should the immigrant need financial assistance.
The Joint Sponsor liability can comes to an end when the intending immigrant becomes a US Citizen.