Apple's Find My Friends app means that if a friend with an itoy is daft enough to accept your request, you can track their location in real time on a map and they can track yours. The iPhone's GPS accuracy is out by much more than 10 metres, so it is no good for finding friends in a crowd.
It is possible for you to set up location-based alerts to tell you when some arrives at or leaves a location. The person you're monitoring isn't told that you are doing this. While if you want to find out it is not difficult. But there's nothing they can do about it except decide to remove you completely.
If all this was not creepy enough, the latest Find My Friends update lets you trigger notifications based on areas rather than specific locations. You simply drag your finger to extend the radius of the area you want to spy on. It means that you can make sure that your ex-girlfriend is not seeing someone else, or that they are not in when you pop around to boil their bunny. [That's the Easter spirit. Ed]