A UnityEditor script that provides a hotkey for quickly grouping GameObjects.

GroupSelection.cs

using UnityEngine;
using System.Collections;
using UnityEditor;

public class ParentSelection : ScriptableObject {

    [MenuItem("OffTheGrid/Group Objects %g")]
    static void GroupObjects()
    {
        GameObject[] objects = Selection.gameObjects;
        GameObject parentObject = new GameObject("Grouped Objects");
        Undo.RegisterCreatedObjectUndo(parentObject, "Grouped Objects");
        foreach (GameObject ob in objects)
        {
            Undo.SetTransformParent(ob.transform, parentObject.transform, ob.name + " Parent");
        }
        Selection.activeGameObject = parentObject;
    }
}