As per question title, how does it work? What are the differences of being in a party or playing alone? Does it apply to finding orbs?
Magic Find is calculated using the Increased Item Quantity(IIQ) and Increased Item Rarity(IIR) of the player that gets credit for the kill. If a minion gets the kill, then any IIQ/IIR from support gems affecting that minion are applied first.
From my experience this also affects opening chest, so when we go out in a party, we usually have the person with the highest MF open the chests.
I think IIQ is the only stat that will give you more orbs, while IIR increases the rarity (color) of an item.
Edit Thanks to Kexlox for finding this information on IIQ in parties:
When in a party, each player in the party after the first gives a +50% item quantity modifier on drops.
Source: Mechanics Thread
2IIQ/IIR ? What's that ?– AntoJan 30, 2013 at 13:30
2Increased Item Quantity and Increased Item Rarity. I'll update my answer to include this.– CoronusJan 30, 2013 at 19:09
2Partying also provides a +50% boost to IIQ per player. So a party of 3 will find twice as many items as a solo player.– KexloxJan 30, 2013 at 19:21
1@Kexlox I was not aware of that. Would you like to submit that as an answer, or do you mind if I incorporate it into mine?– CoronusJan 30, 2013 at 22:50
go ahead, the info comes from pathofexile.com/forum/view-thread/11707 which is a pretty good primer on most of the nuts and bolts of the system– KexloxJan 30, 2013 at 23:44