How to track people by their phone number, using an iPhone, without them knowing?

I've been playing hide and seek with my friends and they are really good at hiding (or I'm just not good at seeking, I've never found them before) They always have their phones with them, so I was wondering would I be able to track where they are by their phone numbers? I would be using my phone (an iPhone 5) and I don't want them to know that this is how I'm finding them. So I wouldn't want their phone to get a notification ect telling them about it. Also something that I don't have to set up with them (otherwise they'll know what I'm doing) Please help!

Work for the NSA… Derp

With a site like http://www.phonecaller.us/ you can easily track down someone using a phone number.

You need either contact their service provider to help you track where their phone is or install a tracking app like find my android/iphone or mobile spy ( http://www.spy-mobile-phone.com/track-mobile-phone-location.html ) on their phone to track where it is.