Here's why you can't do it in Diamond:
* The Underground in Diamond was focused on exploration and collecting. You could find items, battle wild Pokemon, and customize your secret base. However, there wasn't a system for trading items directly with other players.
* Wireless communication was limited. While you could connect with other players wirelessly for battling and trading, there wasn't a feature for sharing items in the Underground itself.
What you can do instead:
* Trade items through the regular trade system. You can connect with other players wirelessly and trade Pokemon holding the items you want to share.
* Play with your friends in the Underground. You can still have fun exploring the Underground with your friends, battling wild Pokemon, and customizing your secret bases.
For future games:
* Pokemon Black and White introduced the "Wonder Trade" feature, which allows you to trade Pokemon with other players around the world, even if you don't know them.
* Later games like Pokemon X and Y added "Friend Safari", which lets you visit your friend's secret areas and catch rare Pokemon.
Let me know if you have any other Pokemon questions!