Marie, for a group of 5, a couple of methods to consider.
As Tim mentioned, using the RER "B" to Paris and then transferring to Metro is one option. The first station you'll arrive at is Gare du Nord, and it's a bit "large" so if you have lots of luggage that may not be the easiest solution.
There's also the Roissy Bus or Air France Bus, which will drop you either at the Opera Garnier or elsewhere in the city, where you can transfer to Metro.
Possibly the least complicated method (especially given the late hour and the size of your group) would be to use one of the airport shuttles, which typically use Minivans. I believe one of these requires passengers to phone when they arrive in Paris; the Van will then pick-up in front of the terminal and drop-off at the Hotel. For further information, have a look at www.airport-shuttle.com/, www.paris-airport-shuttle.com/, www.airportshuttleparis.com/page/ or www.airport-connection.com/ Some of these may have other passengers as well, so will drop-off at several Hotels.
You might check your local Book stores to see if you can find a copy of the RS Paris or France 2009 as details are listed there.
One other thing to keep in mind is that August is about the PEAK travel season in Europe, so pre-booking a Shuttle would be a good idea.
Happy travels!