Google Maps has a feature that allows you to see the current location of your friends.
What you need? Google Maps app installed on your smartphone/tablet. GPS enabled.
Open Google Maps, click on the bottom-left icon (friends). From here, you can add/search friends even from your phone’s contact list. You just need to know their e-mail address.
Your friend is required to accept and give you permission to track him. The app has some privacy options, you can set it to not display your location (make you invisible on the map).
Once everything was configured, you can see your friends on the map. There is a delay, though. You will see their location from a few minutes ago or even hours. Depends when was the last time their smartphones had Internet access.