I do not envy you....
My older DDs are now almost 18 and 17, but at 14 they were not allowed out unless they were at a friend's house, cinema or a party - and only then when I knew the score. DS is 14 but has no desire to be out and about at night - spends time with friends but they are generally on X-Box/PlayStation etc.
However in the summer when they almost 15 (we live in Spain) I did let them out with their friends but only down to the beach/local square etc, but I tended to collect them around midnight. Though it is very different here in that respect. The girls are both allowed out now, younger one tends to be back around 1 and older one whenever, but she's almost 18 and I've not had a single problem with either.
When we're in London though it is a different story. At 14 NO WAY would I have let them out (I wash 14 in London.....I know what can be done!). Now, their lives in the UK tend to be party based so I know that they're not hanging around in parks/play areas etc.
Do you know for sure that the friends are out? I know I was guilty of saying 'it's so unfair, everyone else is allowed out', only to find that my mother called said friends parents to discover that they had all been fed the same crap!!
I would concede a bit on Saturday nights - but only if you know where she is and who she is with??? IS she just 14 or nearly 15 - the difference in my girls between start and end of Yr 9 was enormous. She's got to build up trust. I wasn't strict per se, however they knew that if I said something then their lives would be intolerable if they didn't do/screwed up etc and they never did. Tell her if she shows that she can be responsible then you will give her more freedom. BUt one fuck up and she can go back to being treated like a ten year old....