modified to support group choosing buildDropdown function from bootstrap-multiselects by David Stutz(https://github.com/davidstutz/bootstrap-multiselect)

gistfile1.js

        // Build the dropdown and bind event handling.
        buildDropdown : function() {
            var alreadyHasSelectAll = this.$select[0][0] ? this.$select[0][0].value == this.options.selectAllValue : false;

            // If options.includeSelectAllOption === true, add the include all
            // checkbox.
            if (this.options.includeSelectAllOption && this.options.multiple && !alreadyHasSelectAll) {
                this.$select.prepend('<option value="' + this.options.selectAllValue + '">' + this.options.selectAllText + '</option>');
            }

            this.toggleActiveState();

            this.$select.children().each($.proxy(function(index, element) {
                // Support optgroups and options without a group simultaneously.
                var tag = $(element).prop('tagName').toLowerCase();
                if (tag == 'optgroup') {
                    var group = element;
                    var groupName = $(group).prop('label');
                    var id=$(group).attr('data-id');

                    // Add a header for the group.
                    var $li = $('<li><label class="multiselect-group"></label></li>');
                    $('label', $li).text(groupName);

                    // Add an input for the group to check children elements
                    if(this.options.enableGroupCheck){
                        $('label',$li).prepend($('<input type="checkbox" class="group-checkbox"/>').data('id',id));
                    }

                    $('.multiselect-container', this.$container).append($li);

                    // Add the options of the group.
                    $('option', group).each($.proxy(function(index, element) {
                        this.createOptionValue(element,id);
                    }, this));
                }
                else
                if (tag == 'option') {
                    this.createOptionValue(element);
                }
                else {
                    // Ignore illegal tags.
                }
            }, this));

            // Bind the change event on the dropdown elements.
            $('.multiselect-container li input', this.$container).on('change', $.proxy(function(event) {

                if($(event.target).attr('class')==='group-checkbox'){
                    var groupId=$(event.target).data('id');
                    $('.multiselect-container li input[data-group-id="'+groupId+'"]').prop('checked',$(event.target).is(':checked')).change();
                    return;
                }

                var checked = $(event.target).prop('checked') || false;
                var isSelectAllOption = $(event.target).val() == this.options.selectAllValue;

                // Apply or unapply the configured selected class.
                if (this.options.selectedClass) {
                    if (checked) {
                        $(event.target).parents('li').addClass(this.options.selectedClass);
                    }
                    else {
                        $(event.target).parents('li').removeClass(this.options.selectedClass);
                    }
                }

                var $option = $('option', this.$select).filter(function() {
                    return $(this).val() == $(event.target).val();
                });

                var $optionsNotThis = $('option', this.$select).not($option);
                var $checkboxesNotThis = $('input', this.$container).not($(event.target));

                // Toggle all options if the select all option was changed.
                if (isSelectAllOption) {
                    $checkboxesNotThis.filter(function() {
                        return $(this).is(':checked') != checked;
                    }).trigger('click');
                }

                if (checked) {
                    $option.prop('selected', true);

                    if (this.options.multiple) {
                        $option.attr('selected', 'selected');
                    }
                    else {
                        if (this.options.selectedClass) {
                            $($checkboxesNotThis).parents('li').removeClass(this.options.selectedClass);
                        }

                        $($checkboxesNotThis).prop('checked', false);

                        $optionsNotThis.removeAttr('selected').prop('selected', false);

                        // It's a single selection, so close.
                        $(this.$container).find(".multiselect.dropdown-toggle").click();
                    }

                    if (this.options.selectedClass == "active") {
                        $optionsNotThis.parents("a").css("outline", "");
                    }

                }
                else {
                    $option.removeAttr('selected').prop('selected', false);
                }

                this.updateButtonText();

                this.options.onChange($option, checked);

                this.$select.change();

                if(this.options.preventInputChangeEvent) {
                    return false;
                }
            }, this));

            $('.multiselect-container li a', this.$container).on('touchstart click', function(event) {
                event.stopPropagation();
                $(event.target).blur();
            });

            // Keyboard support.
            this.$container.on('keydown', $.proxy(function(event) {
                if ($('input[type="text"]', this.$container).is(':focus'))
                    return;
                if ((event.keyCode == 9 || event.keyCode == 27) && this.$container.hasClass('open')) {
                    // Close on tab or escape.
                    $(this.$container).find(".multiselect.dropdown-toggle").click();
                }
                else {
                    var $items = $(this.$container).find("li:not(.divider):visible a");

                    if (!$items.length) {
                        return;
                    }

                    var index = $items.index($items.filter(':focus'));

                    // Navigation up.
                    if (event.keyCode == 38 && index > 0) {
                        index--;
                    }
                    // Navigate down.
                    else
                    if (event.keyCode == 40 && index < $items.length - 1) {
                        index++;
                    }
                    else
                    if (!~index) {
                        index = 0;
                    }

                    var $current = $items.eq(index);
                    $current.focus();

                    // Override style for items in li:active.
                    if (this.options.selectedClass == "active") {
                        $current.css("outline", "thin dotted #333").css("outline", "5px auto -webkit-focus-ring-color");

                        $items.not($current).css("outline", "");
                    }

                    if (event.keyCode == 32 || event.keyCode == 13) {
                        var $checkbox = $current.find('input');

                        $checkbox.prop("checked", !$checkbox.prop("checked"));
                        $checkbox.change();
                    }

                    event.stopPropagation();
                    event.preventDefault();
                }
            }, this));
        },

changes on lines: 19, 25-27, 34, 49-53