Well, Mom, it was inevitable, you have come to that crossroad of being a friend vs being the parent. It is a natural progression. Just like she is trying her wing and learning to fly, you have to be there to guide her and let her know you will catch her if she falls.
If you truly have that good relationship, sit her down and tell her SOMEONE SAW THEM... (I am sorry, never let her know you snooped; and, snoop often!) and reported back to you. Tell her that you know she lied to you. Let her know that what her friend did was wrong too. Since you don't want her to know you know her phone number was given out, talk to her about the consequences of being someplace like that at night and possibly meeting up with other people she doesn't know. They might not be the type person she want so associate with but she wouldn't know it until it was too late. Talk to her about her choice of friends. Above all, talk to her about trust issues and let her know that she has tested your trust by her actions. Remind her that SHE is responsible for her actions, not her friend. Let her know if she is ever in a situation she feels uncomfortable with, to make up some kind of excuse, (she can use YOU as an excuse) and call you to come get her.
However, let her know her actions require a punishment. I suggest you start with taking away the cell phone because you know that means something to her. Replace her voice message with your own - telling her friends that your daughter is grounded from her phone right now, so, please do not leave a message at this time. This is not too harsh of a punishment but it lets her know you are still the parent and she is the child and there are rules to be followed.
Give her a chance to explain what happened and how she found herself in that position. Remind her that at her age she will be experiencing different situations; but, she has to make the choice whether to follow the rules or disobey. It is her choice but your job is to guide her and when rules are broken, there are consequences.
YOU will have to remember this conversation - it may be the only time you will ever have to have it; but, if not, you have to be consistent in the future about disciplining her. Parenting doesn't have to be hard if YOU do what is right. Discipline with love and compassion and tenderness; but, discipline and be consistent. Don't back down and don't waiver.
God bless you and guide you.