There is a wealth of accomodation in Benicassim, from hotels, to apartments, villas and campsites. You have lots of choices and it really does depend on your personal tastes and preferences really. If you book early enough and are in a big group, you can pick up some bargains on Benicassim apartments. Some of these apartments can accomodate up to 10 people, the one big advantage of an apartment being that you have air conditioning and a fridge. Very valuable items with the scorching heat that Benicassim provides.
If you are looking for accomodation such as villas or apartments, you can try a holiday rentals site like Holidaylettings.co.uk.
If you decide to camp at Benicassim, you do have a choice in campsites, from Benicamp, FIBcamp and the new Villacamp which is described more as a VIP camp. You can buy camping tickets with your regular festival tickets which turns out to be very cheap as they entitle you to camp for up to 9 days.