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.
Your product “Custom Registration”, does it work with any theme? Is it a plugin or template file? Thanks
Thanks for posting this. I was looking for the signup link as well.
Hello dear,
I have a Multi-Site eHow7.com. Now Need MU theme for Home page where have signup system.
Can you Help me..?
You can use any theme. The signup page is included in multisite. Please look at the url on your install.