Section 6: (Advanced topic) SParameters and multiple driven ports
SParameters assume one port is driven and the rest are terminated if this condition isn't met, then these gain numbers won't directly apply to the observation. However, you can still use SParameters to calculate what will happen when multiple ports are driven. Consider the fourway power divider in Figure 2.5.1, where port 1 is the input (the cable TV signal goes into here) and ports 25 are the output (these signals are sent to TVs around the house). If port 1 is driven, it makes sense that port 2 will have around a quarter of the power coming out (6dB less than the input). But if we turn this around, and Ports 25 are driven, obviously the output from Port 1 is not going to go down to quarter power (6dB gain): if we drive multiple ports, we expect more power coming out then we put into any single port we expect power to be conserved. We expect four times the power of any single port. The problem is we are breaking that SParameter condition. Two approaches can be taken.
Excess Loss
The quick approach is the concept of
excess loss: how much loss do we see beyond the expected power
divider loss. For example, the output of a fourway divider should be
6dB lower than the input. In reality, the device will have some
additional loss in the metal wires and the dielectrics; the actual
divider might measure 7dB. The 1dB difference between the theoretical
loss and the measured loss is the excess loss, and it is the power
burned up inside the device and/or reflected by the inputs (that S_{11} return loss keeps
some power from even entering the divider).
A quick way to approximate the combined power coming out of Port 1 with Ports 25 driver is to simply sum the
power going into the combiner (e.g., 1W into four ports is 4W, or
6dBW) and subtract out the 1dB of excess loss (so 5dBW is at the
combiner port, or 3.2W).
Real Math
Perhaps you want a more exact answer, or don't trust "engineering" math. Doing the math for real isn't hard.
The voltage wave coming out of port 1 (the summed port) is equal to the weighted sum of all of the
incident waves. V_{1}^{+} is equal to zero (this is the output port), and assuming all ports are driven equally then.... that's just the standard power / voltage / resistance relationship.
So, doing it for real really isn't so hard. If your intuition is throwing a red flag, go through the math:

For a fourway combiner, from the port 1 single input to the ports 25 outputs, the transmission Sparameters are S_{21} = S_{31} = S_{41} = S_{51} = 0.5 (SParameters are voltage ratios, and the square of the voltage ratio is the power ratio, so the power ratio is 0.25)

The sum of incident power is S_{21}^{2} + S_{312} + S_{41}^{2 }+ S_{51}^{2} = 1

The incident voltages are 7.07V for 1W incident power and a 50ohm system.

The output voltage wave (V_{1}^{}) is 28.28V

The output power is 4W.
Main Point
Even though SParameters don't individually apply directly when multiple ports are driven, it's easy to get expected power output from the SParameters. Please remember that these voltage waves are complex vectors, keep track of phase as you do the math!