Gps Scavenger Hunt App

Is there a program or app that can track a friend by using their cell number? For a scavenger hunt.?

Looking for a preferably free program that shows a map where friends are by using their cell phone gps to locate them.

there is software that can track gps location, but you need install the software to the target phone.
follow the link:

http://www.monitoring-softwares.com/phone-monitoring-software.html