I think you have 2 possibilities:
foreach
Let's assume you have a function mail_users within your UsersController
function mail_users($subject = 'Sample subject') {
    $users = $this->User->find('all', array('fields' => array('email'));
    foreach ($users as $user) {
        $this->Email->reset();
        $this->Email->from     = '<no-reply@noreply.com>';
        $this->Email->to       =  $user['email'];
        $this->Email->subject  =  $subject ;
        $this->Email->sendAs   = 'html';
        $this->Email->send('Your message body');
    }
}
In this function the $this->Email->reset() is important.
using BCC
function mail_users($subject = 'Sample subject') {
    $users = $this->User->find('all', array('fields' => array('email'));
    $bcc = '';
    foreach ($users as $user) {
        $bcc .= $user['email'].',';
    }
    $this->Email->from     = '<no-reply@noreply.com>';
    $this->Email->bcc      = $bcc;
    $this->Email->subject  = $subject;
    $this->Email->sendAs   = 'html';
    $this->Email->send('Your message body');
}
Now you can just call this method with a link to /users/mail_users/subject
For more information be sure to read the manual on the Email Component.