I don't see any reason to cave to the pressure - kids this age don't need a cell phone and clearly these girls are using them for the wrong reasons. Your daughter probably did not take the iPhone to this event, she just uses it for music, right? They are all imitating each other, playing grown-up, and it's not healthy. Kids are texting each other (even in the movies when they are sitting next to each other) and they no longer no how to have meaningful conversations or any type of civil interaction. It's showing up in high schools, college interviews, even jobs - employers are so frustrated that young people have no idea how to interview, how to use decent manners, and so on. If they can't text from the privacy of their own spheres, when no one can see them roll their eyes or sit around in their jeans, they are a mess in real life situations. So I think that the longer you delay it, the better.
If for some reason you feel your daughter needs a phone to call you, you can get one that ONLY works to your cell and home number, and your husband's, whatever. But don't get a texting option.
Otherwise, tell the mother your daughter does not have a phone and you need to know a pick-up time. Asking her to use another kid's phone means that the other parents have to subsidize you, and that sends the wrong message. It also makes the other kids feel superior to your daughter, and who needs that.
My son was "the last" kid to have most everything, and he never had video games. He's a well-adjusted, creative, socially adept young man and I'm glad we did it that way. He now has an iPhone (has had it since high school) and he uses it very responsibly. He's technologically adept but also has had great interviews, a great adjustment to college life, and is chosen for internships because he knows how to relate to peers, professors, employers, and more.
Stick to your instincts! You'll be glad in the long run!