Select - Custom Default Value

custom.js

instance.component.onSetItems = function(component, items) {
  var employees = [];
  for (var id in component.data) {
    if (
      component.data[id] && 
      (id === 'employee1' ||
      id === 'employee2' || 
      id === 'employee3')
    ) {
      employees.push(component.data[id].data.email);
    }
  }
  return items.filter(function(item) {
    return (employees.indexOf(item.data.email) === -1);
  });
};