Is there a group fare discount from hong kong airport to kowloon? we are 5 people.

For the airport express, I do not know any group discount for 5 people.
Recently, there is an activity concerning a group people of 4. If you are in a group with 4 people, the totally fare to Kowloon Station is 220HK$.
Since you have 5 people, it is highly recommended to take a taxi directly and the taxi fare is around 230HK$.