New blog at registration

We want new registrants to have an option to “get a blog” on registration form (I believe we had this at one point – but it seems to be missing) but now the registration is just for an account to our main site and when they activate it takes them to the admin page with profile to main site. In the network settings, the registration setting is set to allow both sites and user accounts can be registered.

What we want is that they sign up for their site at registration and when they activate it, it will take them to the front-end of their new site. They would also be automatically subscribers to our main site.