If you are a minor without a license, or a permit you will have that privledge prolonged past the time you were supposed to be getting a license/permit. In most states you have to have taken a 30 hour course of drivers education, 6 hours behind the wheel and 30 hours (10 of them being at night) with a licensed driver who has over 5 years driving experience and a valid license. The teen in the car will have nothing more happen to her but a questioning of who she is (depending on state laws about minors, ex. curfews)and if she was aware she was riding with a non licensed driver.
If the minor doesn't have a license or a temporary permit, they shouldn't be driving around period. In addition, depending on the state, even if the minor did have a license or temporary permit, they are not allowed to drive without an adult in the car. Period. What you can do is: 1) Try to pull the minor aside and discuss it - letting them know that essentially what they're doing is probably (highly likely) illegal, so that they can be aware, if they weren't already and stop. 2) If it is absolutely necessary for the minor to be driving, and they have no license, learner's permit or temp license - you can help them apply for a hardship license, which is: -- A license that a minor can have (again depends on what state, but I believe all states offer this option) which allows them to get a license before they've reached the usual legal driving age. In order to get this kind of license all the same rules as a regular license apply (driver's course/driving test etc.) AND the applicant must prove a hardship necessitating early licensure. For example: single parent household with 5 children where the primary provider (a*k*a THE PARENT) has sustained major illness/injury, has little or no readily available support system to help and the eldest child is say 15. The applicant (the 15 year old) would need to prove that he/she HAD to drive for whatever reason (help take care of family): to take younger siblings to/from school, go grocery shopping, etc. You just have to prove the NEED to have a license "early" rather than just a desire to have one early. The situation need not be as serious as the one just described, it simply has to be a REAL NEED. 3) Find the responsible parties (parents/guardians) and repeat option #1. If that doesn't work proceed to step 4. 4) Get the information of the responsible parties (parents/guardians) and TURN THEM IN. Sucks for the minor, as well as the responsible parties (they really should be ticketed/fined), but by now you've tried other options without success, and what they're doing (or allowing the minor to do) is still illegal SO THEY'LL JUST HAVE TO GET THE HELL OVER IT!
If the minor has no license, and you have already tried to contact the parents with no result, I would contact the local or state police and report it.