CDG airport to Gare de Lyon TGV station

The most convenient choice would probably be Le Bus Direct Route 4. Costs a little more than the RER and Metro but a lot easier. Or, if you're a group, a taxi for 50 euros for up to four people.

The most direct service is LIne 4 of Le Bus Direct, serving Gare de Lyon before ending at Gare Montparnasse.

RER trains require a connection at either Gare du Nord or Chatelet les Halles.

Thank you Dick and Laura. Exactly what I needed to know. And it just so happens that we are a group of four! :-)

But the RER connection at Les Halles is as easy as it gets, just walk across the platform.

If you're a group of four, it will cost more to take the Bus Direct than it will to take a taxi, which is fixed price of 50€ for four people. (Bus Direct will be 72€ at €18 times four people.)

Make sure to ignore the taxi touts who will approach you in the terminal and go to the taxi line and get in an official taxi. But you'll save yourself more than 20€, plus the extra time the bus takes to pick up passengers at all terminals before leaving the airport.