How to allow people to create an account to post a classified ad?

I have created a custom role for people who are placing an ad on my site ("Advertiser"). So when someone clicks on "place an ad," they are prompted to log in. That's fine--but in most cases, these people are not going to have an account. So I need to enable them to register/create an account first. Can I have this happen? How do I set this up?