Enforce your rules, you have every right to say no to who comes into your house and no to running her around in your car
Bear with her, it's typical of a teen of her age to do this, it'll probably pass, I was having the same panic myself not long ago
The worst thing you can do is go beserk, she won't trust you to listen to her, talk to her about it, ask her about using, arm her with the facts, let her know it's disappointing that she's choisen to do this, and you won't be supporting her using it in any way (no friends over, no smoking it in the house, no funding it, no taxi service)
Other than that, let her work it out for herself, be there when she's ready to talk, don't look down on her and let her know you are there for her no matter what, the rebellion won't be so appealing when its not getting at you - she's going to do it anyway, so shouting screaming and holering isn't going to do anything but push her closer to the very peope who encourage her to partake, they will become the people who 'understand' her