I'd like to set a default for the customers_allowance field to access one or two of the sites. What would I need to add to the previously used SQL command?
Unfortunately setting a default for the TEXT type isn't possible.

Second question is what SQL Command would I need to enter when someone is given permission to access the more restricted site?
Simply an update query with the correct parameters.

For example (if you add another field in the "edit customer", admin) the field in the form is called "site".

Just do this query then:

Code:
$field_customers_allowance = '--';
$field_customers_allowance = $_POST['site'];
$field_customers_allowance . '--';
mysql_query("UPDATE customers SET customers_allowance = '".$field_customers_allowance."'" WHERE customers_id = '".$_GET['cID']."');
That will be it.