I have never heard of getting on a list with GM. I have, however, heard of getting on a "list" with a dealer. You place your special order through a dealer to GM. So, if you want to get on a list, you go to your dealer, get them to place you on a list, and then the dealer calls you to say they are ready for you to come in to place your order.
Let's say that dealer "A" gets allocated 5 top of the line Camaros per quarter. The dealer has 15 people on a list (in order from 1st person to come in to last). The dealer gets 5 open allocation slots. Dealer calls buyer, says come in, order is placed, cars arrive, and are sold off the lot. Next quarter arrives and GM says you get another 5. Process repeats. I can not say for fact, but I imagine this is how it would run. (This is how I ordered my 02 Z28).
I wouldn't have the slightest clue how and to whom you would contact to get on a list w/ GM. You don't buy the car from them. You buy it from a dealer.
And, like Stovt said, sometimes lists do mean mark ups. If a dealer "A" says we're only selling them at $5K over MSRP right now, then those willing to pay that $5K over get their cars from the list first. As they axe people from the list, they drop their GOUGED PRICES (cause that's how I see it), and call in those willing to pay $2K over MSRP, and so on. Lists can mean mark ups. I, for one, refuse to pay anything over MSRP. I don't care....I'll wait and spend less to get the same as the guy who spends $5K over. That's just $5K less he/she will have to spend on mods!!!