There are already apps in the marketplace that do this kind of thing. Look for phone tracking apps. I have one named Anti DT, which when activated enables me to track my phone either from another copy of Anti DT or from a browser on a PC.
The app will need to contact a central server which can maintain the location of all other users. That would amount to a web service, and it may well use a DB. I don't understand where you see the choice between a DB and a WS - they're complementary.