Seperate /user/register fields and admin/people/create
Probably hook_form_FORM_ID_alter on user_register_form then some logic like 

function MYMODULE_form_user_register_form_alter(&amp;$form, &amp;$form_state) {

  // Is logged in user admin.
  if (user_access('administer users')) {

    // Hide myfield.
    $form['myfield']['#access'] = FALSE;
  }
}