I suggest you use the ApplicationEventHandler.

using Umbraco.Core.Models.Membership;
using Umbraco.Core.Services;

namespace MyApplication
{
    public class UmbracoEventHandler : ApplicationEventHandler
    {
        protected override void ApplicationStarting(UmbracoApplicationBase umbracoApplication, ApplicationContext applicationContext)
        {
            UserService.SavedUser += UserServiceSaved;

            base.ApplicationStarting(umbracoApplication, applicationContext);
        }

        private void UserServiceSaved(IUserService sender, SaveEventArgs<IUser> e)
        {
            throw new NotImplementedException();
        }
    }
}