I've checked with the FAQ, and the information on experience is pre-1.10. Arreat's Summit on the other hand, looks up to date, but when doing the math I can't seem to get it right.
What troubles me is the way experience is divided when in parties. You get 1*(N+1)/2 experience per player in the game, which means +50% xp for each additional player. But, if you party up with those players, and they're the same level as you, the extra experience through increased population should be neutralized, since you xp gained is then factored by [clvl] / [sum of party clvls]. If everyone's level 50, and you're 4 in the party, that's then 250% of single-player xp plus the 35% party bonus (284%), factored by 50/200 = 71% experience, as opposed to the 100% you'd get if soloing.
The problem with this is that it doesn't add up with what I experience through online play. Doing Baal runs with 4 players certainly yields more experience than doing it alone. Time spent is obviously also a factor, but for a start I'd just like to understand the mechanics.