mdb07, I am going to answer "no," you don't need to be there, but it might be more complicated than it seems. From what I can find, and from past experiences, you cannot purchase tickets online with the Pass Member Discount like you can for Busch Gardens. They must be purchased as Regular Admission tickets at the gate for the 50% Off Pass Member Discount.
That said, you might be able to give your friends your pass, and they can simply present it as they purchase their tickets at the gate for the discount. This might be a bit more time-consuming (perhaps get them there earlier?), but I don't believe pass members can purchase discount tickets online. When I did this with my grandparents to get them in, this is what we had to do. Keep in mind that your picture pops up on your pass when they scan it at the ticket window, but I've never seen them check to verify who is using the pass, so you probably won't need to be there. Also, only Gold and Platinum pass holders are subject to this discount, in case that's not what you have. Up to six of your friends can use your pass a day for the discount, too.
If this is too much of a hassle or won't work for you, Kings Dominion is currently selling discount tickets ($20 off) for "Pink Days," but it won't save you as much as the Pass Member Bring-a-Friend. In addition, you can order "Beagle Bucks," which are like Cedar Fair versions of Disney Dollars (in-park money), and save 10% on all in-park purchases.
Wow! That's kind of long! Okay, basically, you won't need to be there, but your friends might have to take your pass and buy the tickets at the gate for the discount. This should work. Hope this helps and that your friends have a great time!
