To introduce an additional layer of security over new trainee registrations, go to your Dashboard and click Manage members > Manage Subscription Plan, then click Manage members. Check Require approval of new members.
Then you can send the invite link to the member, as outlined in this article.
When a trainee signs up for the Premium account, you will receive a notification.
Important: when the "Require approval of new members" box in your Dashboard is checked, your trainees will not be able to access the Premium account until you approve it on the Manage members page.
To approve a trainee's account, click Manage Subscription Plan > Manage Members, then scroll to the "Members awaiting approval" section. You will see the trainees waiting for approval. Click Approve to allow access to the Premium account for the trainee.