In a shared wireless channel, what are the factors that impact channel gain for each user?
The channel gain that is required for a radio system is determined by the distance between the sender and receiver, the operating frequency and the types of antenna used. Other factors include fade margin and the potential background interferences of other non-associated devices. The weather can sometimes play a part especially at higher frequencies.

The data rate also determines the bandwidth of the receiver and higher bandwidths means potentially more incoming noise and therefore, to successfully receive good data, a higher transmit level is needed.

In a multi-user system, assuming that each user is allocated a time slot then the power required to transmit and successfully receive IS NOT determined by the number of other associated users. 


  Can you please explain the channel gain calculation with a simple
  numerical example (say, with 5 users)?


No, because it does not depend on other associated users.