The "Invite Subscribers" module provides a dialogue into which you can paste a large number of email addresses. This then generates an email to each of those addresses. The email contains a link which, if clicked on, subscribes the recipient to your forum.
You need to request the feature so Nabble can keep a record of who has it, as it can be abused by those who harvest email addresses.
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.