Do I earn experience faster in a group?

Generally speaking, if I'm in a full group, will I level up faster than I would if I were playing solo?

This is obviously a broad question with a lot of variables, so let me try to eliminate a few:

• Assume that we progress through content at the same rate in both scenarios.
• Assume that the group always travels together so that everybody gains XP.
• Assume the same number of monsters are defeated in both scenarios.

Given these constraints, if I want to level up fastest, should I be solo or grouped, and why? How will the XP be shared, multiplied, or split (if at all)? Will the monsters the group kills be worth more XP because they are tougher?

The XP is shared. Every player gets the whole XP value for every monster, without splits or multiplication; the only catch is that you have to be within a screen and half of the kill, otherwise no XP for you.

As of patch 1.0.8. Party size affects exp gain positively. You get a buff called "Strength in Numbers". Being in a party will grant you more exp per monster kill.

Players now receive a 10% bonus to Magic Find, Gold Find, and Experience per additional player in a multiplayer game, up to a maximum of 30% in a four-player game