If you use Find My Friends regularly, you can check up on people and where they are. Most people go to a few locations regularly, and you can interact with those specific locations through Find My Friends. Here's how: While someone is at a specific location, you can choose a label for that location. Tap on the person's name in your Find My Friends list, and while viewing the map, tap on "More".
Add labels to friend's locations to set notification alerts
Under "More", tap "Choose Label" and choose from your existing labels or create something new.
Add labels to locations by tapping More
Once you have a label, you can interact with it using the "Notify" option in Find My Friends, or even using Siri. For example: "Notify me when Sally leaves work," or "Let me know when Jimmy arrives at school." When that happens, you get a little notifier on your phone. It's a convenient way to watch for kids or even adults with irregular schedules to help coordinate things on your end.
Tap Choose Label to add or create a new label for a location
Keep in mind this still only tracks the iPhone, not the person, so a forgotten device won't help much! This also gets you a nicer piece of information when you ask Siri where someone is, because now she will tell you someone is "at work" or "at the coffee shop" instead of some address that may not be helpful to you.