If we go 5 persons. do we need to buy 2 group tickets of Hong Kong Airport Express?

Yes, you can buy a group of 2 ticket and a group of 3 ticket. By the way, the group of 2 ticket costs HKD 170 to HK Station; HKD 150 to Kowloon Station; HKD 100 to Tsing Yi Station.
The group of 3 ticket costs HKD 230 to HK Station; HKD 210 to Kowloon Station; HKD 140 to Tsing Yi Station.
You can find the clear fare standards on this web page. :)