To find out what stairs you can currently challenge someone in, first go to the Main Stairs Page, then click on "Show your stairs only" link. The ones in BOLD are ones you can make a challenge in. .
WhisperzQ: the only problem with that is that there are people here that I know of that would slow down in a game that they are losing in to the point that the other game would finish and a new one start and then they would finish the new one still before the original game just to have better odds of wining a second game