As noted, a shuttle service to Geneva is likely to be much more expensive, if you book it
as a private shuttle. If you book it as a shared shuttle, they will try to fill it to capacity, and
you will pay quite a bit less. Probably $40-$50 a person. Or, as I noted, you take the bus,
it will cost you around $20 per person if you buy the ticket now.
If you want to take a shuttle, use findtransfers.com as a way to get quotes from multiple
companies in the Savoie region, who would be a PITA to otherwise contact individually.
The Chamonix-Paris "all-SNCF" route is ~$130-$150 depending on the specific departure.
The Geneva-Paris nonstop train is around ~$70-$75, plus the $20 bus fare. This is the
cheapest option, along with not being an overly strenuous connection setup.
Geneva Airport and Geneva Cornavin are 6 miles apart and a 7 minute train ride between
the two. There are much fewer tracks in the airport station and so much easier to find your
train, but as noted, you will need to change trains at Cornavin for Paris. If you take the bus
to downtown Geneva, you have a moderate walk to the train station. A shuttle can drop
you off right at Cornavin. So I guess it depends on how good you are with walking and
navigating in the city using the bus vs the easier but more $ shuttle or all-SNCF options.
It looks like all the trains go to Gare de Lyon, and you'll have to eventually make your way
to CDG from there.