It sort of is a lottery.
You get to sign-up for 4 unlimited classes but CosCol only guarantees that you get into one class, and if possible, one tour.
What exactly the criteria are for deciding which classes a member gets into I'm not sure but I know that some preference is given based on the order in which they are received (so getting your sheet in as soon as possible is good.)
There are also wait-lists for each class but you aren't guaranteed a spot on the wait lists either. Like I was wait-listed for one class while I was completely out of luck on another.