Tuve esta actividad:

public class MapViewer extends Activity {

    private GoogleMap map;
    private Database db = new Database(this);

    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.mapviewer);

        try {
            map = ((MapFragment) getFragmentManager().findFragmentById(R.id.map)).getMap();
            if (map != null) {
                map.setMyLocationEnabled(true);
                map.setMapType(GoogleMap.MAP_TYPE_NORMAL);
                map.getUiSettings().setRotateGesturesEnabled(false);

                this.addMerchantMarkers(new MarkerOptions());
            }
        } catch (NullPointerException e) {
            e.printStackTrace();
        }
    }

    public void addMerchantMarkers(MarkerOptions mo) {
        SQLiteDatabase dbRead = db.getReadableDatabase();
        Cursor result = dbRead.rawQuery("SELECT title, addr, lat, lon FROM users", null);

        while(result.moveToNext()) {            
            map.addMarker(mo.position(new LatLng(result.getFloat(2), result.getFloat(3)))
                    .title(result.getString(0))
                    .snippet(result.getString(1))
                    );;
        }
    }
}

que cambié de la siguiente manera para usar el agrupamiento de marcadores:

public class MapViewer extends Activity {

    private GoogleMap map;
    private Database db = new Database(this);
    private ClusterManager<MyItem> mClusterManager;

    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.mapviewer);

        try {
            map = ((MapFragment) getFragmentManager().findFragmentById(R.id.map)).getMap();
            if (map != null) {
                map.setMyLocationEnabled(true);
                map.setMapType(GoogleMap.MAP_TYPE_NORMAL);
                map.getUiSettings().setRotateGesturesEnabled(false);

                setUpClusterer();
            }
        } catch (NullPointerException e) {
            e.printStackTrace();
        }
    }

    private void setUpClusterer() {
        mClusterManager = new ClusterManager<MyItem>(this, map);

        map.setOnCameraChangeListener(mClusterManager);
        map.setOnMarkerClickListener(mClusterManager);

        addItems();
    }

    private void addItems() {
        SQLiteDatabase dbRead = db.getReadableDatabase();
        Cursor result = dbRead.rawQuery("SELECT lat, lon, title, addr FROM users", null);

        while(result.moveToNext()) {
            MyItem offsetItem = new MyItem(result.getFloat(0), result.getFloat(1));
            mClusterManager.addItem(offsetItem);
        }
    }
}

Ahora no sé cómo agregar título, fragmento e ícono a cada marcador como el código anterior. Ahora, si hago clic en un marcador, no pasa nada ... ¿Cómo conseguirlo?