Unfortunately, the calculations for dividing transponder power equally amongst users are not as simple as have been suggested.
For the 400mW case :- assume that 400mW is the maximum power that the transmitter can produce. Then a single CW signal could in theory be sent at a power of 400mW, assuming no AGC, beacons etc.
However, if 2 equal level CW signals were being transmitted, then the Peak Envelope Power of these 2 signals would be 6dB higher than each of the signals individually. This is because the voltages of each of the 2 CW signals have to be added together, giving a peak signal with double the voltage of each signal, which is 4 times the power or a 6dB increase.
Therefore, to keep within the 400mW limit, each CW signal would be restricted to 100mW each. The same analysis can be done for 3 equal CW signals, where each would be 9.5dB down; i.e. 44mW each etc.
For SSB signals, the same analysis can be performed by simply taking the PEP of each SSB signal; so one CW signal could co-exist with one SSB signal, both of which would have a downlink power of 100mW.
The above is actually a slight over-simplification, as it takes no account for compression etc, but serves as a useful example.
As Graham mentions, a beacon may be present at a much higher level than the rest of the transponder, which makes the analysis slightly more complex, but the principle remains :-
It's the PEAK power that limits the RF performance of linear transmitters, and that is valid for all linear transmitters.
regards
Grant G8UBN