In order to play the Underground on Platinum, you first need to unlock it. To do this, you must have obtained the National Pokédex from Professor Rowan and have defeated the Pokémon League.
Here is how you can access the Underground and play with friends:
1. Once you have unlocked the Underground, you can access it from the main menu.
2. Select the "Underground" option, and you will be taken to the Underground network map.
3. You will then be able to choose which part of the Underground you would like to explore. There are several different areas, each with its own unique features and Pokémon.
4. To play with friends in the Underground, you must exchange Friend Codes with them. You can do this by talking to them in-game or by connecting with them online via the DS's Friend Code system.
5. Once you have exchanged Friend Codes with your friends, you can select the "Union Room" option from the Underground menu and connect online with them.
6. Once you are connected with your friends, you can explore the Underground together, battle each other, or trade Pokémon.