Tracking someone using their mobile is potentially very dangerous ground. If it is a child (or even an employee during working hours) that's fine and is easy to do, but otherwise, beware!
If you install a tracking device without the owner's knowledge, you could be facing a jail term of between 5 years and life! That's really not worthwhile under any circumstances.
Anyway, legitimately you have several options. Firstly, if the person has an iPhone you can add them to 'family sharing' and track them through 'Find My iPhone'.
If Android is their chosen platform, a similar way to keep track of them is through 'Device Manager'.
Alternatively, there is an app called 'Family Tracker' available on both ios and Android. The person you are tracking will know that the app is on their phone, but you will be able to find out their movements at any given moment.
Some cell phone networks (certainly the major ones) offer family tracking plans. Obviously, these are intended for the safety of children within the family but anyone in the family circle can be tracked.
After that it all gets a little sinister and too much like the movies, so hopefully you will have found something in what I have said that appeals to you. After all, I don't want to have to visit you in jail :-)
