From Geneva Cointrin airport

There is a direct service with mini-buses between the Geneva airport and Grenoble (railway station).
Contact a travel agency for their schedules and price (around 30euros).

Another cheaper solution is to travel by train between Geneva and Grenoble. Take a train from the airport to Geneva Cornavin central station (about very 15 minutes) where you can find a train to Grenoble. However it is a long journey (2h) and the frequency of the trains is low (4 per day).