/**
  * Attach an existing user to an account
  * @param  Account $account Account
  * @param  User $user User
  * @return \Illuminate\Http\RedirectResponse
  */
 public function postAttachExistingUser(Account $account, User $user)
 {
     if ($user->id != $account->owner()->first()->id && $account->guests()->where('user_id', $user->id)->count() === 0) {
         $account->users()->attach($user->id);
         Mail::send('emails.inviteExistingUser', ['account' => $account], function (Message $m) use($user) {
             $m->to($user->email);
             $m->subject(trans('invitation.inviteExistingUser.emailSubject', ['user' => Auth::user()]));
         });
     }
     return redirect()->action('Account\\ConfigurationController@getUsers', [$account]);
 }