Sure. How about adding "This does not require registration in advance." (Or "previous registration" or "a previous account" or whatever is clearest). The problem is users searching for "Create Account" instead of "Sign In" when there is no "Create Account".

Edit: Sorry, on review this post was tangential to the point to which you were responding (about why the email is needed). It was targeted more at the point about user confusion by the login process.