SPWeb.EnsureUser Method throws Access Denied Error

According to Microsoft, SPWeb.EnsureUser  Checks whether the specified login name belongs to a valid user of the Web site, and if the login name does not already exist, adds it to the Web site.

Well, this will not work in an environment where you have membershipprovider enabled. to get around this issue please use the following code. 


#region Add user to a group
        /// <summary>
        /// addUserToGroup
        /// </summary>
        /// <param name="groupName"></param>
        /// <param name="userLoginName"></param>
        public void addUserToGroup(string groupName, string userLoginName)
        {
            SPSite site = new SPSite(RootWebUrl);
            SPWeb web = site.OpenWeb();
            try
            {
                SPSecurity.RunWithElevatedPrivileges(delegate()
                          {
                              web.AllowUnsafeUpdates = true;
                              SPUser spUser = web.AllUsers[userLoginName];

                              if (spUser != null)
                              {
                                  SPGroup spGroup = web.Groups[groupName];
                                  if (spGroup != null)
                                      spGroup.AddUser(spUser);
                              }
                          });
            }
            catch (Exception ex)
            {
                ErrorLogger.LogFeatureError("Error Adding user to SharePoint Group: " + groupName + " Name: " + userLoginName, ex);
            }
            finally
            {
                web.AllowUnsafeUpdates = false;
                site.Close();
                web.Close();
            }
        }
        #endregion



 #region Create user
        /// <summary>
        /// CreateUser - Create a SharePoint User and assign role
        /// </summary>
        /// <param name="strLoginName"></param>
        /// <param name="strEMail"></param>
        /// <param name="strName"></param>
        /// <param name="strNotes"></param>
        /// <param name="strSiteURL"></param>
        /// <returns></returns>
        private SPUser CreateUser(string strLoginName, string strEMail,
                    string strName, string strNotes, string strSiteURL)
        {

            SPUser spReturn = null;
            SPSite spSite = null;
            SPWeb spWeb = null;

            try
            {
                //Open the SharePoint site
                spSite = new SPSite(RootWebUrl);
                spWeb = spSite.OpenWeb();

                SPSecurity.RunWithElevatedPrivileges(delegate()
                       {

                           spWeb.AllowUnsafeUpdates = true;
                           spWeb.SiteUsers.Add(strLoginName, strEMail, strName, strNotes);
                           //Update site
                           spWeb.Update();
                           spReturn = spWeb.AllUsers[strLoginName];
                       });
                //Response.Write("User Successfully Created= " + strLoginName + "<br>");
                ErrorLogger.LogFeatureMessage("User Created", "User Successfully Created= " + strLoginName);
            }
            catch (Exception ex)
            {
                //Response.Write(ex.Message.ToString());
                ErrorLogger.LogFeatureError("Error Creating SharePoint User", ex);
            }
            finally
            {
                spWeb.AllowUnsafeUpdates = false;
                spWeb.Close();
                spSite.Close();
            }

            return spReturn;
        }
        #endregion