Hi there. Thanks for the great question.
When s2Member is running on a Multisite Network, the Open Registration panel that is normally listed under General Options, is instead moved over into the Multisite (Config) panel.
Now, there are two ways to run s2Member on a Multisite Network.
1. Membership Only ( the standard )
2. ( As a Blog Farm ) - this enables a whole new ball game.
If you are using Membership Only ( the standard ), and Open Registration is turned on, then you should see the Registration link on the login page. However, if you configured s2Member to run as a Blog Farm, and Open Registration is turned on, you will NOT see that link, because when s2Member is running as a Blog Farm, all registrations take place under a different registration form.
Instead of: /wp-login.php?action=register ( the standard )
Blog Farms use: /wp-signup.php