borderlands-2
From the question How does auto-pickup work?, we know that ammo is shared between players in co-op. How does this sharing work? Does each player get the full amount of ammo, or is it divided between them? Does it make a difference if one player already has full ammo of the type being picked up?
I'm also interested in the sharing of money and Eridium, but I believe those are covered in How does experience work in co-op?
The stash box is located in the northwest corner of Sanctuary, just past Zed's clinic. To get access to the stash, talk to Claptrap (who is in the same location) and he will give you an absurdly "impossible" mission. Follow the mission to its conclusion, which will point out the position of the stash. You can deposit up to 4 items in the shared stash, which you can remove with any other characters you create.
See also this walkthrough video on IGN.
Ammo and backpack upgrades are acquired on the black market. These purchases have to be made with Eridium; a sought after mineral on Pandora. Once you reach the city of 'Sanctuary', one of the first missions you encounter is entitled 'Plan B'. You will receive 4 Eridium from Scooter, allowing you to make your first purchase at 'Earl's black Market'.
http://borderlands.wikia.com/wiki/Eridium
Best Answer
Each time you pick up ammunition, every member of your party receives ammunition as though they had picked it up. This is different to the way gold and eridium is shared and there is no indicator to show that you have gained ammunition as a result of one of your party members picking some up.
The below is quoted from one of the discussions as a result of testing the behaviour of this mechanic: