That is impossible.
A forum user has to supply an email address themselves and then confirm through a link sent to that email address that they are the legitimate user of that address. Only then is the address added to the "Registered" group.
However, I suspect your intended question is: How can I ensure that only those I approve can access my forum?
In that case the answer is for you to:
1. Change the user permissions so that:
• Those in the "Anyone" and "Registered" groups have the "View", Create_topic" and "reply" permission removed.
• Those in the "Members" group have those same permissions allowed
2. Add the addresses you want to the "Members" group.
Then as soon as your users register they will have the access you define for "Members" but those in the "Anyone" and "Registered" groups see only a screen advising them to seek send a message asking for access.
For some background on this it might be useful if you read my post at:
http://support.nabble.com/Moderating-free-forum-tp6873076p6873871.html
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.