I was reading thru some of the D3 info and came across this
Monster Scaling by Difficulty and Players
Here is how monsters scale per player in each difficulty:
Normal - 75% Health; 0% XP; 0% Damage
Nightmare - 85% Health; 0% XP; 5% Damage
Hell - 95% Health; 0% XP; 10% Damage
Inferno - 110% Health; 0% XP; 15% Damage
One of my big concerns was that I wanted to play thru the first time on my own to check out every nook and cranny, but I was worried about gaining XP. In D2 it was such a huge XP gain to play with a full group that playing thru solo wasnt rreally an option. By reading whats aove it looks like it doesnt matter if theres 1-4 people you'll still gain the same XP. The health only goes up 75% per player so technically a full grp can kill faster, but I can live with that.