In reply to Cerith:
Hello!
It's hard.
If it's a few of you the easiest way is getting a taxi from Reus - negotiate with the driver - dear at around 75 euros (maybe could be negotiated down).
There is actually a bus which goes to Margalef, which you need to get from the bus station at Reus.
It is the Prades Bus initially and then you need to change to a small minivan which drives around the villages, the change is coordinated and the drivers wait for each other.
On the way back the bus passes through the bus stop in Margalef every day (excluding weekends I believe) at 5:50 and is in Reus around 8.
It's a beautiful place with spectacular climbing!