Based on findings by Chexmix in the forum, we get the following table
The bonus above is measured at the time the monster is spawned.
This is not necessarily the number of players at the time it was killed.
Thanks to Ronin Spoilspot for verifying it in both Realm and TCP/IP games.
Arreat Summit has a page that
On that page, the 35% extra bonus was mentioned.
That bonus is per player in the party.
Finally calculations on this page also uses some classic D2 information from
Jarulf's findings. (Particularly for the steps where monsters are between 6-9
levels of a character)
Calculations used here will be based on that page, and classic D2 information
from Jarulf's findings.
Based on version e1.08
For the purpose of calculating experience, specify the character level only
if it is partied AND in the same zone. If a player is not partied OR not in
the same zone, specify 'Non-sharing'.