Yes, you should request a visitor's pass for each visitor. You can find more information here.
Unfortunately outside visitors cannot stay at the hostel. The rooms are exclusively for people working at CERN. However the hostel will be able to give you some addresses for hotels in the CERN/...
It is possible to take visitors on CERN site but you will have to request a visitor's pass for them systematically. You are responsible for them and you have to accompany them at all times.
They can come on site with the car, if they have a visitor's pass and you are with them and you show your ID card to the guards, explaining that they are your visitors.