It is perfectly possible to add an address as a "Member" before a user registers. It means that as soon as they register they immediately have the full rights you want them to have. Providing an administrator knows the a user's email address and trust that person, I strongly recommend any administrator adopt the pre-approval approach when the forum is designed for a small closed group.
That doesn't answer the problem of why they should disappear from the registered list. I'd recommend posting your forum's URL so Nabble can investigate that issue.
The most likely reason for someone not getting the confirmation email from Nabble is that it has been treated by the user's mail service as spam. Do get your user to check their spam folder.
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.