Selection.GetTransforms  

// C# Example
// Creates a new parent for the selected transforms

using UnityEngine;
using UnityEditor;

public class CreateParentForTransforms : ScriptableObject {
[MenuItem ("Example/Create Parent For Selection _p")]
static void MenuInsertParent() {
Transform[] selection = Selection.GetTransforms(
SelectionMode.TopLevel | SelectionMode.OnlyUserModifiable);
GameObject newParent = new GameObject("Parent");

foreach(Transform t in selection)
t.parent = newParent.transform;
}
// Disable the menu if there is nothing selected
[MenuItem ("Example/Create Parent For Selection _p", true)]
static bool ValidateSelection() {
return Selection.activeGameObject != null;
}
}