do airport express have any discounts for more than 2 people traveling?

1)Do they have any discounts for more than 2 people travel together? I will go with my parents but only me have flight ticket.
2)Where can I buy the express ticket in Kowloon? Online or in MTR station?
3)Also my flight is 1st Jan 2013 1am, so I will arrive hongkong one day earlier at 31th Dec 2012. So can I check-in first at kowloon station and drop my bag first but stop in kowloon for shopping for few hours then take the express train to hk airport?