Tap Add, and a list of potential parties with whom you can share appear. It isn't your full Contacts list but, rather, seemingly filtered by those people who have an email address that is also an Apple ID. Select one or more people, and tap Invite, and they will receive an alert and can opt to follow you or not.
Find My Friends used to let you set up both a quasi-permanent group from which you can add and remove people, as well as time-bound temporary events. Temporary or ad hoc groups have now been moved to iMessage, described later.
Where I start to feel a bit of squick is with the geofencing options. Geofencing is a nifty feature that lets you attach an action to moving into or out of a defined geographic area. Of course, this is all opt in: nobody gets alerts about your movements without your explicitly setting it up.
Tapping the Me button and then tapping Notifications lets you set up a list of people with whom you want to give them alerts. Right Now pushes your current place, while When I Leave and When I Arrive sends the note when your state changes. This can be one-time, or, by tapping Repeat Every Time, recurring. To prevent a series of notifications, you can drag the geofence, shown as an orange shaded circle, further away to encompass more variation in location, such as across an entire college campus or large building up to cities or states. The limit is a 1,500-mile radius.
If your children (or perhaps elderly relatives, with permission) have cellular-enabled iOS devices, being able to know when they arrive or leave school, home, or other locations can be a passive way to obtain peace of mind without the necessity to text or call from either end of that relationship.
This works in reverse, too, as anyone sharing his or her location with you can be pinned down as well. Tap a contact in the Friends list, then tap Notify Me, and you can pick to be told when they Arrive or Leave their current location or any location you specify.
You can switch the device that's used for presence, as in the Settings app, by tapping the Me item and picking a device from Share My Location From section near the top.
What it's good for: letting your family members know where you are.
Family Sharing is a more intimate way to give up-to-date information on your whereabouts to those with whom you also trust with your iTunes purchases and other items.
The option, which works across iOS 8 and Yosemite, allows up to six people to share iTunes, iBooks, and App Store purchases (although not all in-app purchases). It also shares information via Find My Friends and Find My iPhone/Mac.
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