Attach the event handler in the init method of your ActiveRecord.

Records.php

<?php
class Records extends \yii\db\ActiveRecord
{
   const EVENT_SEND_EMAIL_UPDATE = 'send-email-to-users';
   public function init()
   {
    // first parameter is the name of the event and second is the handler. 
     // sendMail method from $this class.
     $this->on(self::EVENT_SEND_EMAIL_UPDATE, [$this, 'sendMail']);
     parent::init(); // DON'T Forget to call the parent method.
   }
}