Out of curiosity, why do you say that? I would expect it's as simple as creating a polling/request call to the phone's GPS coordinates. The data can be stored on an application's server and then displayed onto any mapping choice for that person's "group".
I haven't dabbled much in mobile development, so I really couldn't say how hard it overall would be.
I haven't dabbled much in mobile development, so I really couldn't say how hard it overall would be.