It can be used by a group, bur according to the port authority page on it:

Time-based MetroCards

The 10-Trip AirTrain JFK Only Usage MetroCard can be purchased at AirTrain JFK’s Howard Beach and Jamaica stations. Ticket vending machines and the NYC Pennsylvania Station’s ticket vending machines displaying the AirTrain logo. The 10-Trip Air Train JFK will expire 12 months (365 DAYS “ONLY”) after first usage. Every fourth usage, there is an 18-minute waiting period before next usage.

So if you have over four people, you need to wait 18 minutes after each group of four people.

Also note they are saying the expiration is now a year instead of six months.

From http://www.panynj.gov/airports/jfk-cost-tickets.html

I'm sure that's because homeless type people were buying the cards, then offering to swipe people through for $3.50 or $4 each. They get a buck or two, the traveler saves a buck or two, everybody but the MTA wins.