Before you create a multiplayer game you must open the developer console; To do so you must press the following key combination: CTRL+ALT+` (The "`" key is below "ESC" and above "TAB" on most keyboards).

After you have done this enter the following commands into the console separately from one another with the X being replaced with the amount of players you want (Be aware that in the vanilla version of the game 16 is the max you can have):

• sv_maxplayers X
  
• steam_lobby_max_members X

You can now create your multiplayer game, more than 4 people will be able to join and it will show up in the server browser as the number you replaced X with e.g. 16/16.

• Players past the 4th do not have a character standing in the lobby but they are still present.
  
• The game will scale appropriately for the increase in players so you will have extra bosses and stronger elites showing up much earlier.
  
• I highly recommend you use the swarms & sacrifice artifact if you intend to make it more than 1-2 extra players as fighting for chests isn't exactly fun.

Before you do anything you're gonna need to understand the basics of modding so choose where you'd rather read your guide below then return here after you know what you're doing.

Now that you have the basics of modded Risk of Rain 2 you need only install the mod below and then edit the config file or use the console command to change the max player count to 999 if you really wanted to.