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

gistfile1.js

        // Will build an dropdown element for the given option.
        createOptionValue : function(element,groupId) {
            if ($(element).is(':selected')) {
                $(element).attr('selected', 'selected').prop('selected', true);
            }

            // Support the label attribute on options.
            var label = $(element).attr('label') || $(element).text();
            var value = $(element).val();
            var inputType = this.options.multiple ? "checkbox" : "radio";

            var $li = $('<li><a href="javascript:void(0);"><label class="' + inputType + '"><input type="' + inputType + '" /></label></a></li>');

            var selected = $(element).prop('selected') || false;
            var $checkbox = $('input', $li);
            $checkbox.val(value);
            $checkbox.attr('data-group-id',groupId);

            if (value == this.options.selectAllValue) {
                $checkbox.parent().parent().addClass('multiselect-all');
            }

            $('label', $li).append(" " + label);

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

            if ($(element).is(':disabled')) {
                $checkbox.attr('disabled', 'disabled').prop('disabled', true).parents('li').addClass('disabled');
            }

            $checkbox.prop('checked', selected);

            if (selected && this.options.selectedClass) {
                $checkbox.parents('li').addClass(this.options.selectedClass);
            }
        },

ware modified function arguments(added groupId argument) and on line 17 added attribute for group identification