You would need to ensure that your main recordset is edited to join the tables. This is the recordset you are using in the search. The recordset query must join the two tables otherwise you will only ever have an id instead of a username.
When the query is updated you can use the username column binding in place of the current user id. So if your current code for that is displaying the userID looks like this:
<?php echo $row_YourRS['userID']; ?>
You would update it to look like this:
<?php echo $row_YourRS['username']; ?>