Code:

var portalSettings = new PortalSettings(portalId);

var user = new UserInfo();

user.PortalID = portalId;
user.IsSuperUser = false;
user.FirstName = "John";
user.LastName = "Smith";
user.Email = "John.smith@dnndev.me";
user.Username = "johns";
user.DisplayName = "John";

user.Profile.FirstName = user.FirstName;
user.Profile.LastName = user.LastName;
user.Profile.PreferredLocale = portalSettings.DefaultLanguage;
user.Profile.PreferredTimeZone = portalSettings.TimeZone;

var oNewMembership = new UserMembership();
oNewMembership.Approved = true;
oNewMembership.CreatedDate = System.DateTime.Now;
oNewMembership.Email = user.Email;
oNewMembership.Username = user.Username;
oNewMembership.IsOnLine = false;
oNewMembership.Password = UserController.GeneratePassword();
user.Membership = oNewMembership;

if (UserCreateStatus.Success == UserController.CreateUser(ref user))
{
    // User created successfully
    DataCache.ClearPortalCache(user.PortalID, false);
    // send email to user
    DotNetNuke.Services.Mail.Mail.SendMail(user, MessageType.UserRegistrationPublic, portalSettings);
}