Geneva has a very good public transport and CERN is located at the terminus of tram number 18. It is easy to access from anywhere in Geneva.
If you stay in Geneva it is strongly discouraged to use a car. It takes 10-15 minute tram ride from CERN to downtown, while 2 hours by car with no parking.
If you prefer to stay on the French side, use of a car might be an advantage.
Accommodation on the Swiss side
Look for a hotel along the tram line no. 18. The route is shown in this map. CERN would be at the top left corner. If you prefer meetings and social life after the workshop sessions, the Swiss side, closer to downtown might be preferred.
Starting closest to CERN and most far from downtown:
Route de Meyrin, then passing the airport (note the vicinity of the airport)
following a large public transport interchange point Blandonnet
towards downtown along Rue de la Servette
And finally, there is a ton of hotels downtown e.g. around the main train station Gare Cornavin. This will be best for the after workshop activities.
Accommodation on the French side
there are not many pubs or restaurants on the French side, it is a "bedroom". These three hotels are in a walking distance from a bus stop which will drive you to CERN
You will have an opportunity to ask for a car badge to enter the CERN site during the workshop.