if(isset($_POST['addComment']) && $_POST['addComment'] != '')
{
$post_id = $_GET['post'];
$q = mysql_query("YOUR INSERT STATMENTHERE");
}
Then modify your select statement to include the author. Assuming the field is called author in your db
$post_id = $_GET['post'];
$q = mysql_query("SELECT comment, author FROM comments WHERE post_id ='$post_id' ORDER BY comment_id DESC");
while($comment = mysql_fetch_assoc($q)){
.....
}
and now you have $comment['author'], echo it where you want