<select ng-model="selectedGroupToShow.Id" ng-options="g.Id as g.Name for g in myGroups">
    <option value>-- All --</option>
</select>

filter:{GroupId:selectedGroupToShow.Id}"

filter:selectedGroupToShow

<div ng-repeat="item in items| filter:{itemId:selectedGroupToShow.Id||undefined}">
         {{item.itemId}}
</div>