Am i responsible, legally, for minors in my vehicle?:
If they're in your car and you're driving, then yes.  I don't know about Illinois, but in California you're responsible for the passengers in your car minors or not.  If one of your passengers isn't wearing a seatbelt, you can get a ticket in addition to them getting one, even if they're an adult.  When they're not in your car (or even just if you're not driving it) you're not responsible for them unless you and their parents sign something saying you are.