Straight from Finnair's website in 2018:
You have access to the Finnair Lounges and Premium Lounge when travelling on Finnair flights (AY code on the ticket), regardless of travel class. This benefit applies to you and four guests travelling on the same flight. At Helsinki Airport, you are welcome to use our lounges regardless of the operating carrier by showing your Platinum Lumo card and boarding pass. At other airports, the lounge benefit applies only to the lounge printed in your boarding pass and only when travelling on Finnair flights. See all lounge services
It is unclear what is meant by "the lounge benefit" here, i.e. is it the number of guests or Lumo pax own access?