You would have to have to have done it a while ago, but now you can't. Maybe they will change it back some day.
Doubtful that such a day will come..
At one time, you could, but you no longer can. Clicking "Sign up" brings up a page which uses Facebook to authorize your usage of this site.
If you do not have a Facebook account, the only other thing you can do on the page you get when clicking sign up is to join Facebook.
I don't know if that is what the Answers.com originators intended, but that is a change that their programmers have made.
Because of that, you can no longer join answers unless/until you have a Facebook account.
Facebook is becoming a de-facto single-sign-on mechanism.