I also worried about this much. I have submitted my I485 together with my family by myslef. I have been thinking about the supplement of these forms to INS. However, I guess that if someone is permanent resident or USA citizen, then his/her family need I864 form. since that apparently belongs to family-base application.
If he/she is submitting I-485, then their family can submit I-485 concurrently or later but before his/her I-485 was approved, since they are applying for I-485 as derivatives. In this case, that is not family-base application, and so they donot need I-864/134 form. right? Especially, I-134 may usually be used when you want to invit your family or parents coming to USA for visiting not for immigration. Am I right?