first, make sure the name of the select list ends with "[]", for example:
name="select[]"
When you bind the column to the select list, DataAssist will add code similar to:
<?php echo((isset($_POST["select[]"]))?$_POST["select[]"]:"") ?>
change that to use the implode() function to convert the array to a coma separated list:
<?php echo((isset($_POST["select"]))?implode(", ", $_POST["select"]):"") ?>