Adding data to a multi-select lookup field

Adding data to a multi-select lookup field

I have a multi-select lookup field (Email_To) which contains the ID for a table of Persons. I am trying to set the value of this field to the ID of a person record. What am I doing wrong?

    for each sales_enquiry_record in Sales_Enquiries [(Created < "01-Jan-2006") && (Email_To is null)]
    {
sales_enquiry_record.Email_To.add(sales_enquiry_record.Person);
    }