How to add one section separator for Navigation Drawer in Android?

Make sure you define each group with a unique ID, separator won't appear without the ID.

For example, this is my drawer_menu.xml:

<?xml version="1.0" encoding="utf-8"?>
<menu xmlns:android="http://schemas.android.com/apk/res/android">
    <group
        android:id="@+id/menu_top"
        android:checkableBehavior="single">
        <item
            android:checked="true"
            android:id="@+id/drawer_item_timeline"
            android:icon="@drawable/ic_timer_grey600_24dp"
            android:title="@string/drawer_timeline"/>
        <item
            android:id="@+id/drawer_item_reports"
            android:icon="@drawable/ic_timetable_grey600_24dp"
            android:title="@string/drawer_reports"/>
    </group>

    <group
        android:id="@+id/menu_bottom"
        android:checkableBehavior="none">

        <item
            android:id="@+id/drawer_item_settings"
            android:icon="@drawable/ic_settings_black_24dp"
            android:title="@string/drawer_settings" >
        </item>
    </group>
</menu>

Gabriel adds below in the comments that if the group doesn't have an id, the separator will not appear.

To separate menu items by a divider line, only group items with a unique id like following example:

activity_main_drawer.xml:

<?xml version="1.0" encoding="utf-8"?>
<menu xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    tools:showIn="navigation_view">

    <item
        android:id="@+id/nav_apps_and_games"
        android:icon="@drawable/ic_apps_black_24dp"
        android:title="@string/my_apps_and_games" />

    <item
        android:id="@+id/nav_bookmarked_apps"
        android:icon="@drawable/ic_add_bookmark_black_24dp"
        android:title="@string/bookmarked_apps" />

    <item
        android:id="@+id/nav_manage_downloads"
        android:icon="@drawable/ic_downloading_file_black_24dp"
        android:title="@string/manage_downloads" />

    <!-- SET A UNIQUE ID TO THE BELOW GROUP -->
    <group android:id="@+id/group1">

        <item
            android:id="@+id/nav_settings"
            android:icon="@drawable/ic_settings_black_24dp"
            android:title="@string/settings" />

        <item
            android:id="@+id/nav_sign_up"
            android:icon="@drawable/ic_card_membership_black_24dp"
            android:title="@string/sign_up_login" />

    </group>

</menu>

Visual Result: