Copied from the MH forums -
1 hero at level 60 = +30% xp bonus.
2 heroes at level 60 = +20% xp bonus (50% total).
3 heroes at level 60 = +15% xp bonus (65% total).
4 heroes at level 60 = +10% xp bonus (75% total).
5+ heroes at Level 60 = +5% xp bonus for each hero (in addition to the 75% bonus from the first 4 heroes you have at level 60).
Additionally, each level 30 hero grants a 1% XP bonus and each level 50 hero grants a 3% XP bonus.
You can achieve a max of a 200% XP bonus with this system. Your regular bonus xp from items, boosts, etc... isn't factored into the 200% max. This bonus is only useable for heroes at level 60 or below, so if we ever increase the level cap, only heroes at 60 or below will be given this specific xp bonus.
Lastly, I know a lot of you asked for the option to enable and disable this feature, so I went ahead and added that option to the Gameplay Options screen. This will make the December patch as well.