SharePoint 2010 FBA : Add FBA users to SharePoint group programmatically

SharePoint stores FBA users to below format

i:0#.f|sqlmembershipprovider|Surya (please replace with your membership provider name)

 here i is identity provider

      f is membership provider name 

you may get below error when the format is wrong

The user does not exist or is not unique.

   void AddUserstoGroup()

        {

            string siteurl = “yoursiteurl”;

          

            SPSecurity.RunWithElevatedPrivileges(delegate()

            {

                using (SPSite site = new SPSite(siteurl))

                {

                    using (SPWeb web = site.OpenWeb())

                    {

                        web.AllowUnsafeUpdates = true;

                        //string userName = "i:0#.f|sqlmembershipprovider|kumar";

                        //userName = userName.Split('|')[2];

                        string userName = "i:0#.f|sqlmembershipprovider|" + txtEmail.Text;

                        web.SiteUsers.Add(userName, txtEmail.Text,txtLastName.Text + "," + txtFirstName.Text, "");

                        SPUser user = web.SiteUsers[userName];

                        if (user != null)

                        {

                            //TNI Visitors

                            web.Groups["Visitors"].AddUser(user);

                            web.Update();

                            web.AllowUnsafeUpdates = false;

                        }

                        web.Update();

                        web.AllowUnsafeUpdates = false;

                    }

                }

            });

        }