If you are on a limited schedule, ground transport is always risky -- not only during rush hour. As you may know, there are currently street demonstrations in Paris over a new labour law. Traffic is being rerouted, the metro is not.

At CDG, you can buy a tourist transport ticket that will be valid on both the RER and metro. Since you are in a group, perhaps one of you could get the tickets, while the others wait to pick-up the baggage. Once you reassemble, take the RER to Gare de Nord. There you will change to the Metro line 4 (Porte d' Orléans). Line 4 stops in Gare de Montparnasse.