In my time using a mates guest pass I discovered that the fastest way up the ladder is a spot of brown-nosing and getting in there with some guys who're close to but slightly higher than your level. You get quite good experience in parties that way and often you get any drops that the creatures may give because the guys above you simply don't need the stuff. So you find yourself rapidly getting some fairly nifty items (for lower levels anyway) and making some friends along the way who'll happily help you complete quest adventures etc.
I made it up to level 21 using just the guest pass (which expired and though I enjoyed the game, I can't afford a monthly fee just to play it).
