The tab provides a box in which you can insert unlimited email addresses. After filling it with addresses and clicking the "Invite Subscribers" button, Nabble generates an automated mail to all those addresses with a mail that contains a link which people click on to subscribe, i.e. receive messages posted to the forum.
However, it does not register them on your forum, so on most forums - those that only allow registered users to post - they cannot reply. This can create confusion for those not used to forums, and often causes those used to mail lists problems.
Having users post by email can cause other forum administrator problems. Many people top-post replies and never delete original text. This can make forums very difficult to read with longs chains of quoted responses appearing under one line posts posts.
Many people display extremely poor email practice. They are inclined to hit reply on any old message to get a message to a forum and, failing even to change a subject line, post stuff that doesn't relate in the slightest to the original topic. Such practice can make a forum near impossible to manage, making it very difficult for users to look up and find stuff previously posted.
In short, be wary of issuing invites unless you are certain the recipient knows what to expect and you are prepared to hand hold both the generally Internet suspicious and experienced mail list users.
Just a Volunteer Nabble Helper - because the nice folk at Nabble have helped me!