Go to Settings>General>iCloud and in there is Family Sharing. Tap that, then add the Apple ID of the Family Members you want to add, and its as simple as that.
Once its done there, then anything in iTunes, and the Mac App Store will be available, back to a certain date. In the Mac App Store, under the right column where it says, how big a file it is, and the Developer, it will tell you if Family Sharing is available on any App. Some do, some don't, and some do if bought after day x.