You need to use the PHP implode() function to convert the array that is crated by the checkbox group to a comma separated list:
You checkbox group is named "Contact_Me_group_Search_Engine_Optimization", so the code to convert it to a comma separated list would be:
<?php echo((isset($_POST["Contact_Me_group_Search_Engine_Optimization"]))?implode(", ",$_POST["Contact_Me_group_Search_Engine_Optimization"]):"") ?>
You can find more information on the implode function here:
function.implode.php