Most efficient way from CDG to Orsay Museum ?
Efficient how ??? Time ? Money ?
Money first, time second.
RER B from the airport with a switch to RER C at St Michel / Notre Dame. The d'Orsay is one stop down the line. About twelve bucks each. Less than an hour.
If there are four of you take a taxi will work out about same price. Taxi will run 45-55 euros.. the RER is 9.5 euros each..
Will you be carrying luggage? If so, you might consider the museum policy on checking luggage which you can read here under Cloakrooms:
http://www.musee-orsay.fr/en/visit/services/facilities.html