This achievement requires you to earn 1 million dollars in a single game.
There are two ways you can set this up, depending on what browser you're using:
Firefox:
Use the Multifox add-on for the Firefox browser. Install it and each new tab opened (ctrl+T) can be assigned a different profile by clicking the fox icon in the browser in the top right corner.
Google Chrome:
Click on the person image in the very top right of your browser. Go all the way down to "manage people" and click on it, then click on "add person" in the bottom right corner. Once you click it, click an image for them then click "save" in the bottom right corner. This will open a new pop up window for that person.
I'd suggest using the Firefox method as it's easier to navigate tabs than windows, but I couldn't get it to work for some reason so I wen't with Chrome.
Regardless of what method you use, open 4 tabs or windows with these methods and join a game. Select one tab that you want to win every round and enter their name or their answers memorably so you won't forget. You need to ensure you use both votes available on that player, otherwise you won't reach 1 million. After round three, you'll unlock the achievement.