Radius
You should separate the radius and notification distance into two different things. For example I would only want to be notified if a friend was randomly within a kilometre of where i am, but also want to be able to see where others are in a broader radius, say 10km, just out of curiosity to see their whereabouts. Maybe have a sort of distance slider to set notification radius, but keep the Find My friends screen thing youve got going on currently for the radius for curiosity. Cool idea!