I'd take a taxi. It's an easy trip via mass transit -- RER B to Gare du Nord (assuming that you are arriving at CDG) followed by a short walk, but the fare will be 9.75 euros per person. To my mind, with 5 people the cost benefit of the RER would not support the effort but others have different views. Depending on traffic (which I concede is a big if), your fare should range between 45 and 55 euros, but could go as high as 65-70 euros. Don't worry about the size of your party; there are maxi-cabs at the airport and the dispatcher will match your group to a taxi that will hold you and your luggage. These taxis are not more expensive than regular ones. I am not sure if you know but taxis in Paris charge a small supplement for each passenger after the 3rd one and an additional supplement for extra luggage -- I think it is 1 euro per piece for anything over one piece, but I don't know for sure having never taken a taxi with more than 2 pieces of luggage.