You will need to have 2 subforms. When you click the radio button, it will hide one of the subforms and show the other. By default, the forms would both be hidden. In the java script for the "change" event of the radio-button, hide one and show the other. Below should help you get started.
Nothing selected:
"New User" selected:
"Existing User" selected:
Java for radio-button change event:
switch(this.rawValue) {
case '1':
form1.frmMain.frmNewUser.presence = "visible";
form1.frmMain.frmExistingUser.presence = "hidden";
break;
case '2':
form1.frmMain.frmNewUser.presence = "hidden";
form1.frmMain.frmExistingUser.presence = "visible";
break;
}