Unfortunately, the CERN hostel is already over-booked for the period of the workshop. You can give a try but it is very unlikely you'll get a room.
We advise you to book a room in a nearby hotel, e.g. in St Genis/Thoiry
A more complete list of suggested hotels in the area is provided on the CERN housing service webpage. Special rates are usually applied for CERN visitors.
Notice that the main CERN entrance is in Switzerland, while some of the suggested hotels are located in France. Public transportation from locations in Geneva, Meyrin or Cointrin is efficient, but not from hotels located on the french side. Additionally several hotels have shuttles.