Using the mail function, mail(), you can just input multiple addresses in the 'to' parameter. For example:
PHP Code:
mail("bla@bla.com, [email]bla2@bla.com[/email], [email]bl3@bla.com[/email]", $subject, $message);
This would send it to three addresses. The address have to be seperated by commas inside the string (inside the quotes). The space after the comma is optional but I put it there for readibility (lmao is that a word?). Anyways, that should work.