Select user/author from dropdown

The multiselect field uses the same options as the native checkboxes and select fields. Without making any changes to the field, you could use the query API to fetch a list of users as options.

An alternative would be to fork the field and set the options to users. A simple way to achieve this would be to change the constructor in /field/field-multiselect/field/multiselect.php like this:

  public function __construct() {
    $this->icon = 'chevron-down';

    foreach(kirby()->site()->users() as $user) {
      $this->options[$user->username()] = $user->username();
    }
  }