Adds a context (right-click) menu to Unity's scene hierarchy tab that allows for quick grouping of selected GameObjects under a new parent.

GameObjectGrouping.cs

using UnityEditor;
using UnityEngine;

#if UNITY_EDITOR
namespace CelestialStatic.Editor {

    public class GameObjectGrouping {

        [MenuItem("GameObject/Group selected GameObjects", false, 0)]
        public static void GroupSelected(MenuCommand menuCommand) {
            //Prevent executing multiple times when right-clicking.
            if (Time.unscaledTime.Equals(_lastMenuCallTimestamp)) {
                return;
            }

            var parent = new GameObject();
            var selected = Selection.gameObjects;

            foreach (var gameObject in selected) {
                gameObject.transform.SetParent(parent.transform);
            }

            Selection.activeGameObject = parent;
            _lastMenuCallTimestamp = Time.unscaledTime;
        }

    }

}
#endif