res/drawable/btn_addparticipant.xml

<?xml version="1.0" encoding="utf-8"?>

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<layer-list xmlns:android="http://schemas.android.com/apk/res/android">

    <item android:drawable="@drawable/btn_background" />

    <item>
        <bitmap android:src="@drawable/ic_add_group_holo_dark"
            android:gravity="center"
            android:tint="@color/selectable_icon_tint" />
    </item>

</layer-list>