ProximityGroup inherits from Spatial which is essentially a null/empty/origin object. In addition to everything the spatial features it also has additional functions as listed on this page:
http://docs.godotengine.org/en/3.0/classes/class_proximitygroup.html
Alas it's indeed not documented how they work, however with some experimentation you could certainly find out. It's basic explanation says 'General purpose proximity-detection node.' so for starters what we can derive from it is that this node can likely poll it's proximity to other objects in 3D space using the functions it adds on top of whats inherited from the Spatial.
Looking more closely at the documentation it has a member function broadcast(String name, Variable parameters) and a corresponding Signal broadcast(String group_name, Array parameters). Based on that I'm going to wager a guess that it can broadcast it's proximity to other objects in a group corresponding to that group_name, the Array parameters will likely hold the proximity values.
