If you have your forum set to closed registration, you, as the admin, will need to register new members manually.
1) Go to the Admin panel.
1) Click Registration.
1) Type the Username which the member will use to login.
1) Enter the member's email address.
1) Set a Password.
If you want to set this new user as a moderator or administrator, you can do so here.
1) Click Register.
That's it! You now know how to register a new member in SMF.