Pre- 9/11, they were taking as many jumpseaters as they had open seats in the back. I jumped on them several times and always received a warm welcome. I could always count on getting on even if jumpseating with friends - it was pretty much the one airline going in to Vegas that you could count on jumping on with a group of pilots and all of you making it on.
Post 9/11, I haven't a clue, but I would suspect their policy is just as friendly as before.