Hi David, I don't understand what you are trying to do. What do you mean by "add a user"? If you mean to add someone that can use the forum and post messages, then you should use the "Register" link on the upper right corner of the screen (you will see that link only when you are not logged in). So, the point here is that the user should register himself/herself. You can fill in the registration form for the user, but he/she must click on the confirmation link sent to his/her email.
David Beaumont wrote
Whatis the difference between a user and a subscriber?
A subscriber is a user that receives new posts and replies by email. There are a few things you should understand here. This subscription is independent of mailing list subscriptions (we show a warning about this on the subscription page). So if the user wants to post a message that will be sent to the mailing list, the user must be subscribed to that mailing list, not to Nabble's. Another point is that the user doesn't have to register to subscribe to a Nabble application. So any person can go to the Nabble app and click on "Options > Subscribe via email" and follow the instructions.