Possible to save plain text value when entity reference doesn't exist?
Instead of adding an user entity field you can create a normal text field and then use formalter function to make it an auto complete. 

The steps would be as follows.

Create a custom module. 

First you would need to create a menu item to get the list of users to do that you can create a hook_menu function

function module_name_menu() {  
  $items['userlist/autocomplete'] = array(
    'page callback' =&gt; '_module_name_autocomplete',
    'access arguments' =&gt; array('you can define the permissions here'),
    'type' =&gt; MENU_CALLBACK
  );
  return $items;
}


The next step would be to create the call back function.

function _module_name_autocomplete($string) {
  $matches = array();

  // Some fantasy DB table which holds cities
  $query = db_select('users', 'u');

  // Select rows that match the string
  $return = $query
    -&gt;fields('u', array('name'))
    -&gt;condition('c.name', '%' . db_like($string) . '%', 'LIKE')
    -&gt;range(0, 10)
    -&gt;execute();

  // add matches to $matches  
  foreach ($return as $row) {
    $matches[$row-&gt;name] = check_plain($row-&gt;name);
  }

  // return for JS
  drupal_json_output($matches);
}


The last step would be to create a form alter function to 

function module_name_form_page_node_form_alter(&amp;$form, &amp;$form_state, $form_id) {
  $form['field_name']['#autocomplete_path'] = 'userlist/autocomplete';
}


This would change your textfield into an autocomplete field. you would also be able to enter any other name as well.