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. .
In the ratings charts, non-active players disappear after rules as can be read in the FAQ. The argument for this is to keep the charts more accurate, and I think that works fine. This does not happen in the stairs, which means, that in-active players (in this case Line4 games) "fill up" the stairs, which makes it impossible for some players to invite or be invited. In some cases it seems that the stairs are "dead" with many steps occupied by inactive players or players too far apart to invite or be invited. Can the rules for the rating charts and in-active players, be applied to the stairs? I think we would get much more competition, and more games played in that section