populate a text field upon a dropdown change

The easiest way is probably to add the GHIN number to another property of the list options like: rel="<?php echo $row_rs_members['GHIN_number']?>"

Then you could add an onchange event to the list to take the rel attribute from the selected option and populate another form element.

I'd probably add a delimiter to the option value to use to split the first and last name on when the form is submitted to allow you to easily differentiate the first and last name.

This process is going to require some hand coding in both javascript (to populate the number) and php (to separate the first and last names) so I'd suggest a premier support ticket if you aren't familiar with these scripting languages.