Try:
https://github.com/turnermm/preregister/archive/restricted.zip
It struck me that this requirement might make a useful addition to the preregister plugin. I'm afraid, however, that you will have to be the guinea pig. To use this:
1. You have to fill in one or more domain names as indicated in the configuration manager. You only have to enter the major part of the domain name. That is, if you have
mydomain.com and
mydomain.org, you can enter
mydomain. You can enter more than one domain name, using commas to separate them.
2. The new registrant must be registering from your domain(s). That is, the plugin checks the domain from which
the request originates, not the email address.