Some anon discovered in the thread a more useful way to join a game. With this method, you don't have to add the main account.

1- Discover the ID of the room you want to join, if you don't have it yet.
2- Go to the Monster Summer Game lobby.
3- Open your Console in the Developers Tools of your browser.

• Chrome: WIN: Ctrl + Shift + J | MAC: Cmd + Opt + J
  
• Firefox: WIN: Ctrl + Shift + K | MAC: Cmd + Opt + K

3- Type in the console for exampleand press enter.

4- It will direct you to the game. If it fails, the Steam servers are overloaded, try again.


Remember this:

• You cannot join a room that is more than 10 levels higher than the highest level you have completed that day.
  
• If you try to join a new room and you fail because that requirement, your progress will be cleared.
  
• You cannot join again a room that you have left before.