you do something like this.
<menu xmlns:android="http://schemas.android.com/apk/res/android">
<group android:checkableBehavior="single">
<item
android:id="@+id/nav_home"
android:icon="@drawable/ic_dashboard"
android:title="Home" />
<item
android:id="@+id/nav_messages"
android:icon="@drawable/ic_event"
android:title="Messages" />
<item
android:id="@+id/nav_friends"
android:icon="@drawable/ic_headset"
android:title="Friends" />
<item
android:id="@+id/nav_discussion"
android:icon="@drawable/ic_forum"
android:title="Discussion" />
</group>
<item android:title="Sub items">
<menu>
<item
android:icon="@drawable/ic_dashboard"
android:title="Sub item 1" />
<item
android:icon="@drawable/ic_forum"
android:title="Sub item 2" />
</menu>
</item>
</menu>
this code is taken from the Design Library example app made by Chris Barnes https://github.com/chrisbanes/cheesesquare