I’ve been asked this a few times, as it is not obvious from the admin area where users are supposed to go to sign up – either for a user or a new site in multisite.
You can change the registration settings under Network Admin -> Settings. For members of the public, direct to them to yourdomain.com/wp-signup.php.
Yes, this gives you a signup page almost exactly like the one at WordPress.com. 😉 Any attempts to visit the single WP registration link will be redirected to this signup link on the main site.