How to detect several agents within a radius, what is the best and easiest way to do it?
This: -


  I would also want to have objects not carrying an active radio
  transmitter, i.e., no batteries wandering around, just RFID tags or
  resonant metal pieces.


And this: -


  The room may have several entry points, or it could even be an outside
  environment. Imagine I want to know how many people are in my terrain
  which has around 100m of radius.


Are technically incompatible requirements. I suggest you lower your expectations but I hope someone basically proves me wrong.