Below code will make your dropdown multiple select and will display list of slected values from dropdown
<body class="ng-cloak" ng-controller="DemoCtrl as ctrl">
Selected:<p ng-repeat="item in ctrl.person.selected"> {{item.name}}</p>
<form class="form-horizontal">
<fieldset>
<legend>ui-select inside a Bootstrap form</legend>
<div class="form-group">
<label class="col-sm-3 control-label">Default</label>
<div class="col-sm-6">
<ui-select ng-model="ctrl.person.selected" theme="bootstrap" multiple="true">
<ui-select-match placeholder="Select or search a person in the list...">{{$item.name}}</ui-select-match>
<ui-select-choices repeat="item in ctrl.people | filter: $select.search">
<div ng-bind-html="item.name | highlight: $select.search">
</div>
<small ng-bind-html="item.email | highlight: $select.search"></small>
</ui-select-choices>
</ui-select>
</div>
</div>
</fieldset>
</form>
</body>
ui-select library gives multiple attribute for enabling multiple selection
For detail documentation for ui-select multiple selection click here
Hope this helps.
solved Add list of selected items in select menu