Transportation options from Charles De Gaulle airport to Île Saint-Louis.

I don't think that a mini-van would hold 5 couples and their luggage.
I think you should make a plan to split up.

There are flat rate fees for taxis, now.

The fare would be 55 EU to go to the Left Bank, and if there are more than 4 passengers, each will pay 4 EU in addition to the metered fare. No charge for luggage.
Just follow the overhead signs to the TAXI stand, wait in line, and figure out who will go in each taxi or mini-van.
Make sure to hand the driver the complete address of your hotel, including the Postal Code, so there won't be any confusion.

For 10 people I would do two taxis of five people each -- that way there's one extra 4 euro payment in each vehicle (because each vehicle will have one person above the flat-rate four person norm).

Since Ile St Louis is in the 4th arrondissement, the flat rate is 50 euros for the taxi, plus four euros for the extra person. So that makes it a little less than 11 euros per person.

Make sure to IGNORE the guys who will walk up to you in the airport terminal saying Taxi, Taxi?? and follow the signs to the taxi stand line.

A small correction, since the hotel will be on the Left Bank - you will pay 55 EU for each taxi.

Go to the taxi stand and see how they sort you out. You will save money if you can get 5 in each taxi, so aim for that.

I always take the RER B, but I travel solo and have only a small shoulder bag. With luggage and four people, or more, a taxi makes more sense.

Ile St. Louis has an 04 address and thus is on the right bank for transport purposes. This one is a no brainer. It costs 10.30 apiece to take the train and 50 Euros for 4 and 54 euros for 5 in a cab. No reason not to have 4 people in one cab for 50 and the 3 other couples ie. 6 in a van cab for 58. No reason to split a couple. It would cost 6 people nearly 62 Euro to go by train lugging luggage and then finding the hotel on foot and of course being exposed to pickpockets while exhausted. It will cost 58 door to door by cab. Be sure to only use the official taxi queue and wait your turn; have one person let the dispatcher know you are 6, so he can be prepared to direct a van cab for you.

Write the address including postal code on a piece of paper to give the cabbie and put 50+8 (58)Euro on the slip. Tips are not necessary. If the driver is particularly helpful with luggage etc then a small tip is appropriate. I'd give 60 Euro in this case.

Same for the cab with 4 address plus 50Euro on the slip.

Wow! Thank you all for the input! This was my first posting on the forum and I now realize what a valuable asset it is. Great information which I can pass along to our group. Thanks again!

It is possible that you would get a cabbie trying to assert that Ile St. Louis is on the left bank to get that extra 5 from you. Thus having the address with postal code is crucial; if he asserts 55 as the base fare just point to the postal code ending in 04 of your hotel. The basic fare should be on the meter at the start; don't let them run the meter. They then add the additional 8 euro for two extra people at the end or they could do it at the beginning but there are not other charges. No drop charge, call charge, luggage charge etc -- it is 50 and then 8.