Does observing an EM wave take energy from it? Here is the question that got be started on this: Suppose 1,000 people were sitting in a room with their laptops out and a WiFi transmitter in the front of the room. If everyone was connected to the WiFi would the people in the back be able to connect? Assume that the signal strength is passable but not powerful at the back of the room.
I believe this comes down to the question of whether or not taking a reading, detecting, and ultimately even observing an EM wave takes energy from it. If each laptop in the room was reading data out of the wave then would the strength of the wave be any more decreased than if the wave simply passed through that space?
If there is another underlying concept then please enlighten and I'll alter the question title accordingly.
 A: Please keep in mind one is talking of electromagnetic waves. Take the analogy of visible light projected from the location of  your WIFI transmitter. Light impinging on objects is either absorbed (energy removed from the wavefront) or reflected. There exist shadows, less light behind somebody etc. So in general energy is absorbed from an electromagnetic beam.
Now the WIFI is being absorbed by the antennas of the individual laptops but that is a very small part of the energy of the beam, . Most of the beam will be hitting the walls and going through because, unless metal, they are mostly transparent to the WIFI low frequencies. Shadows will be in the room only if the people are wearing metal clothing.
A: The question how much receivers could receive a radio wave depends from the circumstances. Supposing that the farest away located receiver could receive the signal and this happens in a direct way without reflections from walls and all other receivers do not shadow each other, then YES, all receivers get signals.
This happens because you are working with radio waves. In an antenna electrons get accelerated and decelerated periodically and this led to the emission of polarized photons. The antenna generator and the related rod length are responsible for the frequency of your radio wave. The information you receive is - roughly spoken - coded on this frequency. A receiver has not have an antenna rod of the size of the emitter antenna, it is enough to have a tiny piece of metal and the right electronics behind. The electronics amplify the carrier frequency of the antenna and filters out the information.
