You do this from within the Ventrilo client itself.

1) Log into your server and become the Server Admin.
2) Right-click the lobby, go to \"Server Admin\" and select \"User Editor\".
3) Enter the user\'s ventrilo username, password and select if you want them to automatically become an admin when they log in.

Now, when that user wants to connect to your server, they MUST use the password you assigned them (that is set in their Server Connection Editor, in the \"Password\" field).