Please consider adding new user approval (moderated registrations) to Defender. For organizations that only have a finite set of people who allowed to register, it would completely eliminate spam registrations and make sure only “real members” of the organization have accounts.
The new members would register as usual and get the email verification link to use. Once they have completed that verification they would get an email stating their account is waiting approval, and the admin (or designated member checker) would get an email that there was a registration waiting approval.
This functionality is included in the free plugin https://wordpress.org/plugins/all-in-one-wp-security-and-firewall/ but since I use Defender Pro I’d hate to install another security plugin just for that one feature.
I had been looking at membership plugins but the more I think about it the more this specific use (limited list of people who can register) seems more suited to a security plugin.