Q:

Do you earn less XP per character with a larger party?

I noticed that the game got significantly easier after I found a few more people to join my party. I'm at five party members now, and just using as many as possible seems like the best option right now.
I'm wondering though if there is a drawback to using the maximum number of characters in your party? Does the party size affect XP gain in any way? Is there any reason to use less than the maximum number of characters in your party?

A:

As of Patch 1.03, you gain an additional 10% XP for every empty slot in your party. (i.e. 5 party members = +10% XP, Solo = +50% XP, etc.)
Keep in mind that there is a level cap, and substantially more XP in the game than is needed to reach it. Keep in mind also that the higher level you are when recruiting NPC's, the more decisions in their development will have been made - often poorly. 

