Many Data from Different Transmitters to Single Receiver Simultaneously
There are several ways to get many to one reception in RF, and it's usually referred to a X division multiplex.

There's Time Division Multiplex, TDMA. Somehow, each transmitter knows the time (perhaps a clock, perhaps a receiver), and transmits for its individual burst time.

There's Frequency Divison Multiplex, FDMA. Each transmitter is given a relatively narrow channel. The receiver looks at them all wideband, or tunes to one at a time. GSM/PCS uses a combination of these.

There's Code Division Multiplex, CDMA. Each transmitter is given a spreading code. This is how GPS works. At the receiver, the incoming signal is correlated with each code to separate the signals. It does have the so-called Near/Far problem, where two transmitters at different distances from the receiver must power control to be received at a very similar strength. Not a problem with GPS, but was trouble in IS-95.

And then there are all possible combinations of the above. 

All of these allow all transmitters access to the medium simultaneously.

Less organised schemes can do Carrier Detect and Collision Avoidance, where each transmitter listens for a quiet time to transmit, and can tell if its transmission got stomped on, and try again.