How does X% Chance of Finding Magical Items work for a party?

As of the 1.04 patch on 8/21/2012, each party member gets their own magic find and gold find value applied to the items that drop for them.

Prior to 1.04, magic find in a party was averaged between party members. So if you had a 4 person party, one had 40%, one had 20%, and the other two had none, everyone would have an effective magic find of 15% for their own drops.

The same thing applies to gold find bonuses and bonuses to XP.

For single player with a follower, you get your own value, plus 20% of the bonuses on your follower's gear.

Source

Gold and Magic Find (GF/MF) bonuses are individual (no longer shared or averaged in a party).^{1 2}

Solo, 20% of your follower's MF, GF, and +Exp bonuses stack atop your own.³