Class1.cs

using System;
using System.Collections.Generic;
using System.Text;
using System.Threading.Tasks;
using Autodesk.Revit.ApplicationServices;
using Autodesk.Revit.Attributes;
using Autodesk.Revit.DB;
using Autodesk.Revit.UI;
using Autodesk.Revit.UI.Selection;
namespace Lap1PlaceGroup
{
    [Transaction(TransactionMode.Manual)]
    [Regeneration(RegenerationOption.Manual)]
    public class Class1 : IExternalCommand
    {
        public Result Execute(ExternalCommandData commandData, ref string message, ElementSet elements)
        {
            //Get application and document objects
            UIApplication uiapp = commandData.Application;
            Document doc = uiapp.ActiveUIDocument.Document;
            try
            {
                //Define a reference Object to accept the pick result
                Reference pickedref = null;

                //Pick a group
                Selection sel = uiapp.ActiveUIDocument.Selection;
                GroupPickFilter selFilter = new GroupPickFilter();
                pickedref = sel.PickObject(ObjectType.Element, selFilter, "Please select a group");
                Element elem = doc.GetElement(pickedref);
                Group group = elem as Group;

                //Pick point
                XYZ point = sel.PickPoint("Please pick a point to place group");

                //Place the group
                Transaction trans = new Transaction(doc);
                trans.Start("Lab");
                doc.Create.PlaceGroup(point, group.GroupType);
                trans.Commit();
                return Result.Succeeded;
            }
            //If the user right-clicks or presses Esc,handle the exception
            catch (Autodesk.Revit.Exceptions.OperationCanceledException)
            {
                return Result.Cancelled;
            }
            catch (Exception ex)
            {
                message = ex.Message;
                return Result.Failed;
            }
        }
        // Filter to constrain picking to model groups, Only model groups
        // are highlighted and can be selected when cursor is hovering.
        public class GroupPickFilter : ISelectionFilter
        {
            public bool AllowElement(Element e)
            {
                return
                    (e.Category.Id.IntegerValue.Equals((int)BuiltInCategory.OST_IOSModelGroups));
            }
            public bool AllowReference(Reference r,XYZ p)
            {
                return false;
            }
        }
    }
}