Send an invite using the campaign system.
With this plugin you can send a campaign which requires the “Confirmation” url. Once a mail is sent, the subscriber is marked blacklisted. When they click the confirmation URL they are removed from the blacklist and will be a normal confirmed subscriber in the system.
More info in the README: https://github.com/phpList/phplist-plugin-invite
It may at times be useful to allow sending to subscribers who are already marked unconfirmed in the system. This will need some discussion first.