I have purchased 3 copies of the game for the family and everyone is having a blast. So much so in fact, that we decided that we would like an extra/guest account.

I have read that its possible to use one account and log on additional characters, but I am not clear how that works. It seems that you still need an extra game key regardless.

Is there a way to purchase a key through the account page (i did look and didn't see anything) or do you absolutely need to purchase an additional physical copy (and keep key, throw away everything else, which seems EXTREMELY wasteful!)?