function my_em_bookings_table_cols_template_eventid($template, $EM_Bookings_Table){
    $template['event_id'] = 'Event ID';
    return $template;
}
add_action('em_bookings_table_cols_template', 'my_em_bookings_table_cols_template_eventid',10,2);
 
function my_em_custom_booking_form_cols_eventid($val, $col, $EM_Booking, $EM_Bookings_Table, $csv){
    if( $col == 'event_id' ){
        $EM_Event = $EM_Booking->get_event();
        $val = $EM_Event->event_id;
    }
    return $val;
}
add_filter('em_bookings_table_rows_col','my_em_custom_booking_form_cols_eventid', 10, 5);