Is user logged in

if (!is_user_logged_in())

Get Logged In User ID

$user_id = get_current_user_id();

Is user an administrator?

if (current_user_can('administrator'))

Does user exist?

  $user = get_user_by('email', $email);      //Returns WP_User object on success, false on failure
  if ($user !== False)
  {
    //----- THIS USER IS ALREADY A MEMBER -----
    $UserId = $user->ID;
    
  }