Taxi from CDG to hotel near opera house?

I don't ever take the taxi but since no one has weighed in... To get a taxi, go to the taxi stand outside the terminal. There is a set rate, it used to be 50 euro for right bank and 55 for left bank. I don't know if that's changed. The set rate includes a certain number of people, I think four, and bags. I believe your group will fall within that range. Hopefully someone who chooses the taxi option will weigh in with more certainty. I travel alone - with a small shoulder bag only - and ride the RER B into Paris, then transfer to the metro to get walking distance from my lodging. For a group with bags, the taxi is probably the best choice.

Brad's standard fare is still the same.

If there are three of you, it will still be slightly cheaper to get the non-stop Roissybus from the airport to the opera house itself. Since it's non-stop, it won't be much slower than a cab. The fare is 13.20 euro per person.

The fixed rate to a single location on the Right Bank is 50€. This rate covers 4 people and their luggage. There is no need to tip but some people do. Do not accept rides from anyone who approaches you in the terminal. Follow the signs for taxis and get in the line outside your terminal. I have waited 10 minutes but that was for a vehicle that could fit a group of 6. Usually, the wait is less than 5 minutes. If you want to pay by credit card be sure to check before getting in that the driver accepts them. Unless you are comfortable with your French language skills, I suggest that you write the address of your destination, including the postal code, on a piece of paper and hand that the driver with 50€ also written on it. That does two things: let's the driver know the exact address of the destination and also that you know the flat rate.
Edit: Do you mean the Paris Opera House near Opera metro (Palais Garnier) or the modern Paris Opera House near Bastille? Either way the taxi fare is the same but the Roissy bus is better for Palais Garnier. Also, many hotels in Paris have opera in their names or say that they in the Opera area but really they are not or are not near the single stop for the Roissy bus so if you are not using the door-to-door service of a taxi, it is worth checking a map to see if your hotel is near the Rue Scribe stop of the Roissy bus.

JHK's advice covers it. You can use Google Maps to pinpoint the location of your hotel. I hope it is the Bastille opera you are near because that's a very lively area, both north along the St-Martin canal and east to Gare de Lyon.

For three people to the Right Bank (which is your case with a hotel near the Opera), it would be 16.67 euros per person for a taxi - just 3 euros per person more than the bus. And you'll get to go straight from your terminal to your hotel, rather than making a tour of the airport to pick up passengers at each terminal before ever leaving the airport - and then needing to walk from the bus stop to find your hotel. Plus you'll all get to sit together!

Take the taxi. You can save money other places. Not having to hassle with transportation and baggage at the end of a long flight is well-worth a few extra euros.

Tacking on to this - there are taxi's that accommodate 5 people and their luggage or will we have to break into two groups?

There are mini-vans which can accommodate 5 people or more. Luggage should fit in the trunk, or you may have to hold your purses or carry-ons.

I'd take a taxi in the OP's situation and I have tried just about every way other than hitchhiking to get between CDG and Paris. I concluded that, with the fixed rate, taxis are the best choice if the budget allows that. However, for those wanting to take the Roissy bus or future readers of this thread, I am adding a link that lists the locations of the stops: http://easycdg.com/roissybus-paris-cdg-de-gaulle-airport/ hoping that will help people find the bus stop upon arrival at CDG. There's not much of anything that I can post here that will reduce the chances of smoking near the stops or the less-than-scenic ride into Paris.

I doubt the taxi into Paris takes a more scenic route than the bus. An odd complaint at any rate.