Disclamers:
Only one person can play with mouse & keyboard. The rest have to use game controllers.
Remote Play Together is apparently not available in China, so if you are located there you will need to do step 6 as well. (远程一起玩显然在中国不可用)

Guide:
1. Press Shift + Tab in-game to open the steam menu
2. Open the "Friends" menu with the button on the bottom of the screen if it is not already open
3. Right-click on a friend
4. Press "Remote Play Together"
5. In the Steam Remote Play menu, make sure to "Allow" the friend to use the type of input they are on (Controller or Mouse/Keyboard)
6. If you are located in China, you will first need a VPN, and then right click on the game in your library, go to launch options and type -forceInternationalMode. This is because Steam Remote Play is banned in China

We are working on proper online multiplayer support but it will take a while.