CreateUniqueUser.cs

using Sitecore.Owin.Authentication.Configuration;
using Sitecore.Owin.Authentication.Identity;
using Sitecore.Owin.Authentication.Services;
using System;
using Microsoft.AspNet.Identity.Owin;

namespace Foundation.Authentication
{
   public class CreateUniqueUser : DefaultExternalUserBuilder
   {
      public CreateUniqueUser(string isPersistentUser) : base(bool.Parse(isPersistentUser)) { }

      /// <summary>
      /// Overrides the CreateUniqueUsername function to hook into how Sitecore creates users when you authenticate with Middlware.
      /// </summary>
      /// <param name="userManager"></param>
      /// <param name="externalLoginInfo"></param>
      /// <returns></returns>
      protected override string CreateUniqueUserName(Microsoft.AspNet.Identity.UserManager<ApplicationUser> userManager, ExternalLoginInfo externalLoginInfo)
      {
         IdentityProvider identityProvider = this.FederatedAuthenticationConfiguration.GetIdentityProvider(externalLoginInfo.ExternalIdentity);
         if (identityProvider == null)
            throw new InvalidOperationException("Unable to retrieve identity provider for given identity");

         //While you can map properties in the Sitecore.Owin.Authentication.enabler.config file, you also have the ability to analyze and assign 
         //user profile fields during the user's creation at this stage as well.

         //This could be useful if you have certain computed properties or other factors to look at - for example taking two standardized claims (from your transformations) and then
         //performing some sort of string operation on them, etc.
         
         //Note: You can also access custom claims in here that you set programmatically in IdentityProviderProcessor.cs
          var username = "";

         //Iterate through our claims that were transformed, and set our sitecore username to be the value from the one entitled 'user_name'
          foreach (var c in externalLoginInfo.ExternalIdentity.Claims)
          {
             if (c.Type.Equals("user_name"))
                 username = c.Value; //This is just an example to illustrate programmatically working with claims
           }

         string domain = identityProvider.Domain; //This comes from the <domain> tag inside of your provider configuration file - you could also just set this to whatever you want
         return domain + "\\" + username;
      }

   }
}