How do I add items while in my friends game?

[Jazzu]

I've been trying the add option on the monitor in my friends game for items I've purchased, and it refuses to add them to my game. Am I missing something?

[Kittenpox]

The description for this would be a tad long, and I'm not at my PC (so please bear with me), but:

First, make sure nobody else has added stuff - if they have (such as there being more than one flashlight), first get them to remove things.
Main window in a lobby -> go right from the main pin-board, to the shopping screen.
At the top, switch to Loadouts, then switch to the ALL loadout in the bottom-left.

Press the Buy button on that loadout to purchase all the available items (I have that part set to auto-buy, personally), and then try pressing Add.

Going back to the main screen, where you would press Ready, see if there are any items with markings that have been underlined.
If you can see those, you are able to add items to others' games.

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Ideally you'll want to use Loadouts 1,2,3 to personalise what you choose to add - and that step involves using the +/- buttons by each equipment icon to define what the loadout contains, and then adding them to the lobby.

Hopefully someone else can explain this better; or if not, there may be Guides available here on steam for how to use the shop.