Join My Multisite plugin provides each site in WordPress multisite with three new options for new memberships on their sites:
- Auto-add users
- Have a ‘Join This Site’ button in a widget
- Keep things exactly as they are
If you choose first option i.e. Auto-add user then existing users will automatically get added to your subsite (where this feature is on) as soon they visit that site. If you choose Join This Site option they they for joining site clicking of this button becomes required. Third option is clear itself. If registration is turned off in any subsite then register button will not be visible on that site.
Install and Network Activate Join My Multisite plugin, configuration is done per-site via a page in the ‘Users’ section.