Hello,

Here is very great plugin: wordpress-mu-secure-invites.

Only if a user has email invitation or invitation code, the registration will be shown up, or It will hide the register page behind the layer of Invitation interface, and it works in my wordpress 4.8.1 Multisite although it is old plugin.

For more information, please check at: https://wordpress.org/plugins/wordpress-mu-secure-invites/

If multisite-auth is activated, the user will keep staying at the secure invite interface after click the links at email invitation. that means they will can not see the registration page.

is that possible to make it work together? thanks.

Alex