ESRI Javascript API get features in extent

Question

Based on this part of your "limited question" above:

"I am trying to get the geometries of graphics that are grouped"

I assume you are using the esri ClusterLayer as described here: https://developers.arcgis.com/javascript/jssamples/layers_point_clustering.html

If so then right here we go:

Mouse over will not be ideal at all the reason is that the cluster images are simply PictureMarkerSymbols which are set against a new esri.renderer.ClassBreaksRenderer at different break points for example:

var renderer = new esri.renderer.ClassBreaksRenderer(mapManager.pointFeatureSymbol, "clusterCount");
var blue = new esri.symbol.PictureMarkerSymbol("images/BluePin1LargeB.png", 32, 32).setOffset(0, 15);
var green = new esri.symbol.PictureMarkerSymbol("images/GreenPin1LargeB.png", 64, 64).setOffset(0, 15);
var red = new esri.symbol.PictureMarkerSymbol("images/RedPin1LargeB.png", 72, 72).setOffset(0, 15);

    renderer.addBreak(0, 2, blue);
    renderer.addBreak(2, 200, green);
    renderer.addBreak(200, 1001, red);

You can click on the image rather and get the graphics inside by doing something like the following:

Now assuming you have a cluster layer object somewhere: lets say var clusterLayer = new ClusterLayer

you can attach an event

dojo.connect(map, "onClick", function (evt)
{
        var mp = evt.mapPoint;
        var cl= map.getLayer("Points");

        for (var g in cl.graphics)
        {
            var graphic = cl.graphics[g];
            if (graphic.geometry != null && graphic.geometry.type == "point")
            {
                var toleranceInPixel = 10;

                // now create a buffer around the current graphic and basically build                a polygon for it and get its extent
                var extentPoly = pointToPolygon(graphic.geometry,  toleranceInPixel);

                // then see if this buffered point polygon contans the mp
                if (extentPoly .contains(mp))
                {
                    // add all points that are within your buffered polygon to some array and access their geometries from there
                }
             }
        }
});

Method to build a polygon from point - Note there may be updated esri libraries which has this build in

  function pointToPolygon (point, toleranceInPixel)
  {
    //calculate map coords represented per pixel
    var pixelWidth = mapManager.map.extent.getWidth() / mapManager.map.width;

    //calculate map coords for tolerance in pixel
    var toleraceInMapCoords = toleranceInPixel * pixelWidth;

    //calculate & return computed extent
    var extent = esri.geometry.Extent(point.x - toleraceInMapCoords,
                   point.y - toleraceInMapCoords,
                   point.x + toleraceInMapCoords,
                   point.y + toleraceInMapCoords,
                    map.spatialReference);


    return (convertExtentToPolygon(extent, map.spatialReference));
}

Method to build an extent from polygon - Note there may be updated esri libraries which has this build in

  function convertExtentToPolygon (extent, spatialRef)
  {
    var xmin = extent.xmin;
    var xmax = extent.xmax;
    var ymin = extent.ymin;
    var ymax = extent.ymax;

    var topLeft = new esri.geometry.Point(xmin, ymax, spatialRef);
    var topRight = new esri.geometry.Point(xmax, ymax, spatialRef);
    var bottomRight = new esri.geometry.Point(xmax, ymin, spatialRef);
    var bottomLeft = new esri.geometry.Point(xmin, ymin, spatialRef);

    var rings = new Array(topLeft, topRight, bottomRight, bottomLeft, topLeft);

    var newPolygon = new esri.geometry.Polygon(spatialRef);
    newPolygon.addRing(rings);
    return newPolygon;
}

You can adjust the tolerance as needed - there may be newer esri libraries to do this better but this is similar to what used some time ago.