Really appreciate the fact that you help so many people on a daily basis for seemingly small matters.
We a family of 4 and are landing at Charles De Gaulle airport at 8 AM and we have to got to our AirBNB near Bussy Saint Georges Station. I understand that the destination station is on RER A. How can we travel from CDG to Bussy Saint Georges via train
To go from CDG airport to Bussy Saint-Georges you'll be taking the RER B then RER A train lines.
I'd recommend visiting either CDG T2 or CDG T1/T3 train station ticket window/office to buy tickets directly from an agent so they can create you the proper RER ticket. The regular "to Paris" RER ticket is not sufficient to get you to Bussy St. Georges because you'll be going from Zone 5 to Zone 1 (central Paris), then back out to Zone 5 (Bussy). A regular "to Paris" ticket only is valid for the Zone 5 to Zone 1 part of your trip.
Once you've got these tickets purchased for your family (if kids are 9 or under, they can get a reduced rate) from the RER agent you'll be using these single tickets for your entire trip. (Tip: if you're going to make this trip in reverse to get back to CDG, you can buy each of your family members 2 of these tickets and save/use the 2nd one for your return trip. These tickets have no expiration date so can be used at any time in the future. Just be careful not to mix up the used & unused tickets.)
The route is RER B from CDG (direction Paris) until Chatelet les Halles. Then transfer for RER A in direction Marne la Vallée, which will pass & stop at Bussy St. Georges. Keep those custom RER tickets throughout your trip. You'll need them to unlock turnstiles/gates at Bussy St. Georges.
The total travel time is about 1h30m. Cost of each adult ticket is 16.45€
There is another option that can be much faster, and a bit more expensive although requires more precise timing on your part. You can take a high speed train from CDG T2 TGV station to Disneyland station (Marne la Vallee) then take an RER A train two stops back towards Paris to arrive at Bussy St. Georges. Total travel time is about 10 minutes for the high speed train from CDG to Marne la Vallee, then about 5 minutes walking transfer to RER A for a 6 minute ride on RER A towards Paris / Bussy St. George two stops away. Total cost is about 23€ (19-20€ for TGV train, 2.85€ for RER A). Again, you can buy a custom RER ticket for Marne la Vallee to Bussy St. Georges from RER station ticket agents at CDG. (It's a lot easier than trying to create those yourself using the automated ticket machines.). Or you can buy the RER tickets at Marne la Vallee if you're pressed for time at CDG to catch your TGV/Inoui/Ouigo high speed train (any of these three "flavors" of France national railway high speed trains will work).