They sell a special Octopus card which is a roundtrip journey on the Airport Express. Definitely available at the airport.

The Travel Pass has 3 days of unlimited MTR travel on certain lines.

You can continue to add value to your Octopus cards at any MTR station.

Taxi is cheaper for 4 people but probably won't fit all your luggage. Instead, just take the Airport Express to the Hong Kong Station (more comfortable and faster). From there take the free hotel shuttle H1 to Conrad, it runs every 24mins from 0620-2310.