Apologies for the delay!
The code Pedro added was to a now deleted forum.
However, it seems I copied it to my forum at:
http://ruston-house.1088553.n5.nabble.com/template/NamlEditor$ViewFile.jtp?file=ppc:ppc.naml
where I find the "Invite subscribers" module is installed and, I believe, works but haven't tested it recently.
I must have forgotten how I did it earlier and have done a more customised install here:
http://sbc-forum.2368949.n4.nabble.com/template/NamlEditor$ViewFile.jtp?file=custom_tweak:addforcedsubscriber.naml
As you see here the "Invite Subscribers" module still needs to be authorised by Nabble.
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.