Is user logged in

if (is_user_logged_in())

Get Logged In User ID

$user_id = get_current_user_id();   //Returns 0 if not logged in

Is user an administrator?

if (current_user_can('administrator'))

if ( (is_user_logged_in()) && (current_user_can('administrator')) )

if ( (!is_user_logged_in()) || (!current_user_can('administrator')) )

Does user exist?

  $user = get_user_by('id', $user_id);  //Returns WP_User object on success, false on failure
  if ($user !== False)
    echo "User exists";

  $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;
    
  }