Pokémon Mystery Dungeon: Rescue Team DX is a revamped and updated version of the classic Mystery Dungeon games, bringing updated features, new Pokémon and much more. You play as a person who is transformed into a Pokémon and must save and help your fellow Pokémon while uncovering the mystery of what happened to you. You travel through randomly generated dungeons, recruiting Pokémon and completing quests.
Pokémon Mystery Dungeon: Rescue Team DX is a single player game and doesn't have what you would consider traditional multiplayer. As you play through the entire campaign on your own, you can go on a quest to save a friend stuck in a dungeon. You can also do this for foreigners, via the post office in the city.
Go to the post office, then choose "Help a friend" from the options. If you choose to help a friend via the Password option, you will need their password to access the rescue mission. If you choose the Internet option, you will be given a list of all available rescue missions and can choose which one you want to continue.
The notice board will show you the rescue team that needs help, where they are, the difficulty, the rewards, and the number of days left for that particular rescue mission. You can then charge up and find the team that needs help, then get your reward when you take them out of the dungeon.
So it's not really multiplayer, but it's a way for the community to interact and engage with each other.
