How does Risk of Rain's multiplayer scale?

Specifically:

  1. Do enemies spawn more frequently?
  2. How does money scale (if I kill one enemy in single player, is that money split between 2 people in multiplayer or does each person receive the single player bounty)?
  3. Do enemies have more health?
  4. Do items and crates appear more often?
  5. What items have shared effect between players? (I've noticed that some items provide bonuses for all players, but am not sure if this is across the board or just on certain items)

1. Do enemies spawn more frequently?
The enemy spawn rate is unaffected by the amount of players.

2. How does money scale
Whenever a player picks up some money, everyone in party receives the money.
Whenever someone spends their money on anything, it comes out of their own pool of money, without affecting other players.

3. Do enemies have more health?
No, the health of enemies is unaffected by the amount of players.

4. Do items and crates appear more often
No, and this is the main reason why coop is not easier with more people. The item and crate spawn rate remains the same regardless of how many people are playing and when someone picks up an item, no one else can pick it up anymore and only this player will receive the effect of the item (with some exceptions mentioned next).

5. What items have shared effect between players?

The following items work out of the box for all players:

  • Gasoline
  • Smart Shopper
  • Will of the Wisp
  • 56 Leaf Clover
  • Happiest Mask
  • Ceremonial Dagger
  • Life Savings

The following items work for all players, depending on certain criteria:

  • Bustling Fungus - The bustling fungus won't activate for all players, but if a player activates his/her Bustling Fungus (stand still till shrooms) then other players can stand on it and receive the effect. The effect is limited to the %HP Regen that the Fungus holding player receives.

  • The Hit List - All players see the marks and can kill marked targets to proc the bonus, but only Hit List holders receive the tally marks and bonus damage.

  • The Warbanner - Only the banner holder's level up will activate a banner, but once a banner is posted, all players can receive its benefit.

  • Infusion - All kills activate the benefit of Infusion, but only players holding infusions will gain HP.

  • Meat Nugget - Only the nugget holder's attacks will generate nuggets, but all players can pick them up to receive the health benefit.

  • Snake Eyes - Only the dice holder gains the crit bonus, but all shrine fails increase the tally.

  • Time Keeper's Secret - All players experience the time stop, but only the player who holds the item can activate.

Source