Group the selected objects under a new parent object in Hierarchy (Unity 3D)

EmptyParentCreator.cs

using System.Collections.Generic;
using UnityEditor;
using UnityEngine;

public static class EmptyParentCreator
{
	[MenuItem( "GameObject/Create Empty Parent", priority = 0 )]
	private static void CreateEmptyParent( MenuCommand command )
	{
		// This happens when this button is clicked via hierarchy's right click context menu
		// and is called once for each object in the selection. We don't want that, we want
		// the function to be called only once so that there aren't multiple empty parents 
		// generated in one call
		if( command.context )
		{
			EditorApplication.update -= CallCreateEmptyParentOnce;
			EditorApplication.update += CallCreateEmptyParentOnce;

			return;
		}

		Transform[] selection = Selection.transforms;
		if( selection.Length == 0 )
			return;

		List<Renderer> renderers = new List<Renderer>( 8 );
		Bounds bounds = new Bounds();
		bool boundsInitialized = false;
		for( int i = 0; i < selection.Length; i++ )
		{
			if( AssetDatabase.Contains( selection[i].gameObject ) )
				continue;

			renderers.Clear();
			selection[i].GetComponentsInChildren( renderers );

			for( int j = renderers.Count - 1; j >= 0; j-- )
			{
				if( boundsInitialized )
					bounds.Encapsulate( renderers[j].bounds );
				else
				{
					bounds = renderers[j].bounds;
					boundsInitialized = true;
				}
			}
		}

		Transform newParent = new GameObject().transform;
		newParent.position = bounds.center;
		//newParent.position -= new Vector3( 0f, bounds.extents.y, 0f ); // Move pivot to the bottom

		Undo.RegisterCreatedObjectUndo( newParent.gameObject, "Parent Selected" );
		for( int i = 0; i < selection.Length; i++ )
		{
			if( AssetDatabase.Contains( selection[i].gameObject ) )
				continue;

			Undo.SetTransformParent( selection[i], newParent, "Parent Selected" );
		}

		Selection.activeTransform = newParent;
	}

	private static void CallCreateEmptyParentOnce()
	{
		EditorApplication.update -= CallCreateEmptyParentOnce;
		CreateEmptyParent( new MenuCommand( null ) );
	}
}

How To

Simply create a folder called Editor inside your Project window and add this script inside it. Then, after making a selection, simply click the GameObject-Create Empty Parent button.