Hong Kong Airport Transfers and Octopus card

Hi,

I am travelling a group of 4 and need to travel from airport to Yau Ma Tei. I understand that there is an airport express (AE)ticket of 4 for HK150. Must I purchase 4 Octopus cards too?

I saw this from the website and confused what I should buy. Is it free too for group tickets? Please advise.
"Passengers using Octopus Card to pay for travel on the AE can enjoy free travel on the the whole of the MTR network (including the mainland border stations of Lo Wu and Lok Ma Chau) to or from any AE Station provided all connections are made within one hour of each other"

Some other questions:
1. The airport transfer group tickets are valid on the day of issue only as according to the website. How about return tickets from Yau Ma Tei back to airport? Where do I buy it if I can't buy on the same day at customer service centre?
2. How and where do I claim back the deposit of Octopus card? Is it available at the customer service centre at the airport?

Thanks.