public class UserTemplateExtender : Dynamicweb.Extensibility.TemplateExtender 

public class UserTemplateExtender1 : UserTemplateExtender
{
    public override void ExtendTemplate(Dynamicweb.Rendering.Template template)
    {
        template.SetTag("MyCustomTag", "MyCustomValue:" + User.Name);
        if (IncludeParentGroups)
        {
            GroupCollection parentGroups = new GroupCollection();
            parentGroups.AddRange(User.Groups);
            //Do some actions with groups.
        }
    }
}