monsters give +50% experience per extra player in the game, up to 4.5x normal
they also give +34% experience per extra allied player splitting experience while in range, up to 3.38x normal
then experience is split between allied players in range, proportional to experience levels- a level 5 and level 45 in the same group receive 10% / 90% of the xp split.
these effects are multiplicative with each other and +% xp from anni/shrines/ondals (which are additive with each other)
the normal 'most experience' way to play is to kill monsters in a game with 7 other players you aren't allied with, thus receiving 4.5x overall before anni
however with some difficulty the most possible is to be a level 98 character with 7x level 1 allies in range, in which case you'd receive 14.2x experience overall due to the bonus from allies
the problem with this is that even receiving just 1/105 share of the experience, level 1 characters will instantly level up and soon you'll have level 20+ allies
and once your allies are splitting more of the experience than the +34% bonus each gives (at 7x level 33), you're winding up with less xp overall. Necessitating that you'd somehow have to continuously replace your level 1 leecher bots. and even if you somehow had an unlimited supply of level 1 hellrushed leecher bots to follow in diablo runs and rapidly switch chars, you still run into both the maximum xp cap per kill and overflow glitch, and you definitely can't do it in normal cs mobs because they level up so fast.