The cheapest way in. The Private transfer to Sintra is 37 minutes quicker for €49.05 more, which is not worth it at this party size.
Metro and CP train to Sintra
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 Sintra — 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. CP's own trains to Sintra run about 05:41 to 23:31 on weekdays, with a gap from roughly 01:01 to 05:41 overnight, every 15 to 30 minutes. The metro itself runs 06:30 to 01:00; treat 22:30 as the safe last departure from the airport, since a late metro leaves too little time to change at Alameda and again at Rossio.