Determine if this should be a group
You could use hook_form_alter() to change the field type for the field you want to hide to "hidden". The code would be something like this:

mymodule_form_alter(&amp;$form, &amp;$form_state, $form_id) {
  switch($form_id) {
    case "user_register_form":
      $form["fieldname"]["#type"] = "hidden";
      $form["fieldname"]["#value"] = $form["fieldname"]["#default_value"];
      break;
  }
}


This function will get called for every form displayed, so the switch statement makes sure it's the right form. That being the case, the next line changes the type of the field to a hidden field. Finally, it copies the default value of the field into the actual value (hidden fields don't have a default value).

Unfortunately I don't know what the name of the field you want to change is, so you'll have to investigate that.

Hope this gets you on the right track.