Higher boosts for larger party sizes.

[bloody941]

from what i can tell based on a 50m dps, your exp per hour increases as your party members increase.

1 player (50m dps) - optimal tier 99 - 307m base exp per hour (but being a solo idiot you don't get your 50% party ascension)
2 player (100m dps) - optimal tier 124 - 276m base exp per hour
3 player (150m dps) - optimal tier 149 - 391m base exp per hour
4 player (200m dps) - optimal tier 169 - 399m base exp per hour

the values and what member count may vary as the dps changes, but its already optimal if you are playing with a party that has similar dps to your own. only at max inferno or major walls would i expect to see much of a major difference in the exp everyone gets