S2Member used to fully support WordPress Multisite but I guess that has changed at some point.
When I go to its Multisite settings I see nothing (no options). Not sure if it's a bug or not.
Now there's readme-ms.txt in the plugin's directory which states.
= Is s2Member compatible with WordPress Multisite Networking? =
Multisite support is no longer included in the s2Member Framework. However, it is available with s2Member Pro. s2Member Pro is compatible with Multisite Networking. After you enable Multisite Networking, install the s2Member Pro Add-On. Then, navigate to `s2Member → Multisite (Config)` in the Dashboard of your Main Site. You can learn more about s2Member Pro at [s2Member.com](http://www.s2member.com/).= I was using the free version in a Multisite Network before. What happened? =
s2Member (when running on a Multisite Network) requires minor alterations in WordPress core that are not compatible with plugins available at WordPress.org (i.e., not allowed) at this time. For this reason, full support for Multisite Networks is now available only in the pro version.= What if I have already configured Multisite Network options on this site? =
If you already customized s2Member's Multisite Network configuration options in a previous release, those settings will remain and still be functional over the short-term; i.e., the functionality that makes s2Member compatible with Multisite Networking is still included, even in the s2Member Framework. However, the routines that deal with core patches, and those that allow you to change Multisite options are no longer available. You will need to acquire the Pro version to regain that portion of the s2Member Dashboard._Or, you can revert to [a previous release](http://s2member.com/release-archive/) of s2Member if you'd like. s2Member Framework v151218 is suggested if you go that route._
Here's how to enable the registrations again with a WordPress filter.
You can add it to your functions.php OR even better in a custom plugin or in mu-plugins plugin.
// S2 has a broken MS UI and I have to do this?!?
add_filter("ws_plugin__s2member_check_mms_register_access", function () { return 'user'; } );
