// subcontractors for the job are saved as a test string in the database:
// e.g. 1,2,3,4,5

string subcontractor = (string)dt.Rows[0]["subcontractors"];
String[] subcontractors = subcontractor.Split(',');

for (int i = 0; i <= subcontractors.Count - 1; i++)
{
    // add the subcontractor name to the ListBox based on it's ID from the array
}