Yup! That documentation is out-of-date. Here is the new version for your reference.
This is how the game stage (GS) of a PARTY is calculated:
The GS of all players in the party are calculated.
The players are sorted by GS.
The highest GS number is multiplied by "startingWeight".
This then loops down the list and "startingWeight" is multiplied by "diminishingReturns" every time.
The final result is then converted to an INT that rounds down.
Example:
Players with GS 120, 30, 60, 91, 5, 1, 2, 80
startingWeight= 1, diminishingReturns=0.5 (defined in this xml file below)
So we get
120 * 1.0000000 = 204
91 * 0.5000000 = 45.5
80 * 0.2500000 = 20
60 * 0.1250000 = 7.5
30 * 0.0625000 = 1.875
5 * 0.0312500 = 0.15625
2 * 0.0156250 = 0.03125
1 * 0.0078125 = 0.0078125
...for a total of 279.0703125 which is then rounded down to a GS of 279