I hope you already found the answer, but maybe someone else will find this useful. The previous answer refers to a common drop down not a button drop down. Here is an example, but without the benefits of hg-option while I didn't use a select but a button.
<div class="input-group">
<div class="input-group-btn" ng-class='{open: open}'>
<button class="btn dropdown-toggle"
data-toggle="dropdown"
ng-click='open=!open'>
Action<span class="caret"></span></button>
<ul class="dropdown-menu">
<li ng-repeat="choice in choices"
ng-click="setChoiceIndex($index);$parent.open =!$parent.open">
<a href="#">{{choice}}</a></li>
</ul>
</div>
<input type="text" ng-model="choices[index]" class="form-control">
</div>
where choices is an array of strings that will be displayed in the drop down and index is another variable in the controller scope that will reflect the selected choice.