public abstract class GroupTemplateExtender : Dynamicweb.Extensibility.TemplateExtender 

using System;
using System.Linq;
using Dynamicweb;
using Dynamicweb.Modules;
using Dynamicweb.Modules.UserManagement;

Namespace CustomModules1
{
   public class GroupTemplateExtender1 : GroupTemplateExtender
   {
       public override void ExtendTemplate(Dynamicweb.Rendering.Template template)
       {
           template.SetTag("MyCustomGroupTag", "MyCustomGroupValue:" + Group.Name);
           if (IncludeGroupsAndUsers)
           {
               GroupCollection subgroups = new GroupCollection();
               subgroups.AddRange(Group.Subgroups);
               //Do some actions with subgroups.
               UserCollection users = new UserCollection();
               users.AddRange(Group.Users.Where( u => u.Active && u.ValidFrom <= DateTime.Now && u.ValidTo >= DateTime.Now));
               //Do some actions with users.
               if(Group.Parent != null)
               {
                   //Do some actions with parent.
               }
           }
       }
   }
}