Devise\Users\UserManager::registerUser PHP Method

    public function registerUser($input)
    {
        // if no group_id in input, default to "Adminstrator"
        $input['group_id'] = array_get($input, 'group_id', 2);
        if ($user = $this->createUser($input)) {
            $this->generateActivateCode($user);
            $this->message = 'User successfully registered. An email has been sent with an activation link.';
            return $user;
        }
        return false;
    }

 /**
  * Executes registerUser method in UserManager which
  * attempt to register a new user via pulbic register form.
  *
  * @param  array  $input
  * @return Response
  */
 public function requestRegister($input)
 {
     if ($user = $this->UserManager->registerUser($input)) {
         $this->SessionsRepository->sendActivationEmail($user);
         return $this->Redirect->route('dvs-user-register')->with('message-success', $this->SessionsRepository->message);
     }
     return $this->Redirect->route('dvs-user-register')->withInput()->withErrors($this->UserManager->errors)->with('message-errors', $this->UserManager->message);
 }