How do you make use of the free week by using someones login/code?
I think you use a promo code, say "Sloth" for example, which is a fine code to use :) , then I think they ask you to put in your credit card info, name, etc...to register, then I think they give you open access to the site for a full week.
Now, this would work the same way for pretty much any promo code, but there's something special about using "Sloth". Something different. Something awesome.
so after the week would it start billing me?
Yes, unless you cancel it, which is very easy to do.