Many people have difficulty finding such a sponsor. Here are a few tips that can help.
Although the joint sponsor can't combine income with the petitioner, the joint sponsor can combine income with other members of his or her household as long as they have the same principal residence as and promise to financially support the intending immigrant(s). These additional household members will use Form I-864A.
If the income level is still insufficient, the sponsor can count his/her personal assets and/or the assets of additional household members as described above. Assets are counted such that every $5 of assets counts as $1 of income. For example, if income were short of the minimum requirement by $100, it would take $500 to make up the difference. These assets must be reasonably available to support the sponsored immigrant(s), meaning that they could be converted to cash within one year without undue harm to the sponsor or his or her family members. An attorney can help you decide which kinds of assets would qualify. (For a spouse or child of a U.S. citizen, it's every $3 of assets for $1 of income).
Note that these methods -- counting income of other household members and using assets -- work for the petitioner as well. The petitioner can even use the assets of the intending immigrant(s) under certain circumstances even if the intending immigrant(s) income might not be countable (see below). So maybe the joint sponsor won't even be necessary.
If multiple people are immigrating through the same petition, each immigrant can rely on a different joint sponsor. However, each joint sponsor must qualify independently. They cannot combine their incomes unless, as described above, the joint sponsors are members of the same household.
**Remember, though, that the income of the intending immigrant(s) can never be used to meet these requirements unless it comes from a lawful source AND it can be established that it will continue to come from that source after immigration. Also, the intending immigrant would have to be currently living in your residence unless it is the petitioner's spouse, in which case the current residence would not matter.
These are general remarks only. Check with an immigration attorney for help specific to your case.