There are 2 ways to add a character to a game.
First, you must have Created a Character. Then, after you've created a game, you may navigate to the game's page by using the Games menu item in the top bar, and then selecting the game you'd like to add the character to. For this example, our game is "Example Game", and the character is "Example Character".

This method does not require you to make a character first (though if you did, you are given the opportunity to select one). For this method, enter your email address in the "Invite a Player" box. This will send an email with a link to join the game.
You must use the same email address that you intend to use to sign in or the code will be returned as invalid. If your friends sign up with a different account, you'll need to resend the invite to that email address instead. (We are working to allow you to sign up with a different email than the invite).