On click of checkbox send email to selected recipients using module
"to" parameter in drupal_mail is a string. You can have as many receipent as you like in the the emails "to" string, provided they are separated by commas (",").
You can do like this

$to  = 'one@email.com, two@email.com, three@email.com';
drupal_mail('$module_name', $key, $to, language_default(), $params = array('username' =&gt; 'test'), $from = NULL, $send = TRUE);


You may check the documentation of drupal_mail.