The cheapest way in. The Private transfer to Cascais is 35 minutes quicker for €53.05 more, which is not worth it at this party size.
Metro and CP train to Cascais
CP · Metropolitano de Lisboa and CPPickup, hours & luggage
Boards at. Aeroporto station, inside the arrivals building
EUR 0.50 for a reusable Navegante Ocasional card, if you do not already hold one from the metro fare, plus EUR 1.90 for the metro's own single on that card and EUR 2.55 for CP's single on to Cascais — EUR 4.95 the first time, EUR 4.45 on a card already loaded. The same card pays for both legs; CP does not take the metro gate's contactless bank-card tap.
Luggage. No stated limit on either the metro or CP's trains.
Hours. The metro itself runs 06:30 to 01:00 and CP's own trains to Cascais nearly as late (05:30 to about 01:30), but a late metro leaves too little time to change at Alameda and again at Cais do Sodré before the last one; treat 23:00 as the safe last departure from the airport for this chain.