In reply to Cyrees:
The bus company is Hispano Chelvana, and they normally only run to Chulilla on school days (18:30 from Valencia), so not on PH or weekends. (https://transviago.com/autobuses-hispano-chelvana/)
From Chulilla to Valencia the bus starts at 07:35 from the main square, but again, only on school days. In either case, you need to call them 24-48h in advance to let them know you want to grab it, or they won't stop at Chulilla.
So for your friends, there are two options:
1) Find a bus to Llosa del Obispo, and hitch-hike from there (or arrange a pick up from El Altico hostel if that's where you are staying). Last time I did this, and started walking to Chulilla (45-60min), but quickly got a lift from a local driver.
2) Find a train to Lliria and arrange a taxi from there (40€ or thereabouts).
Once you are there, a car is not required. I've been twice and the longest you walk is about 45-50min to the Pantano area (Chorreras/Oasis, etc).
If you're there for a short time, a car might make sense, but if you're there long time, it'd be very costly.
For any other info feel free to get in touch. If you need help with bus times to Llosa, let me know and i'll give you a hand.
Post edited at 22:14