we are just travelling the above route and return,then metro to blvd bonne nouvelle.is it best just to buy a ticket(5adults) at CDG?
Yes, likely just single-use RER tickets would be best, but let me make some assumptions and precisions.
I'm assuming on arrival day your group will be going from CDG to Gare du Nord and then transferring to Metro line 4 to get to Blvd Bonne Nouvelle area (closest Metro 4 station would be Strasbourg St. Denis), in one uninterrupted trip. If so, a single RER B ticket per person would cover both the RER B & transfer on Metro 4.
The same in reverse as well: If you're going from Strasbourg Saint Denis station, Metro 4, to Gare du Nord and transferring directly onto RER B, then again a single CDG-Paris ticket will cover this trip.
Since there are 5 in your party, you could purchase a booklet of ten tickets (a carnet) at CDG Airport train station ticket offices/windows and use half for the inbound (to Paris) and the other half for the outbound (to CDG) journeys. Buying the booklet gives a slight discount (~20%) and there's no expiry date on the ticket usage.
Please post another question if I've made some incorrect assumptions or you need further clarification. Have a great trip.