If you mean what do you get when you post your URL at the
Invite Subscribers Activation Request area. That allows you to get Nabble to generate emails to multiple addresses that invite the recipients to click a link to subscribe to a particular forum or sub-forum.
You can use that facility to invite absolutely anyone to subscribe, not just users of your forum. As a result it has the capacity to be used by spammers hence needs to be requested manually.
While the facility may have some benefit for you, as you suggest that your audience is primarily focussed on using a forum, it is probably better simply to encourage your users to subscribe by having a pinned post in a prominent areas of your forum and a description of that explains the benefits of subscribing to it - plus, of course, a set of posts there that give a clear indication of the emails they would receive.
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.