This is the official app from valve.

---------- Post added at 07:02 AM ---------- Previous post was at 07:01 AM ----------

"Those who wish to join the beta just need to download the Steam app (free of charge) and log in to their Steam account via the mobile app to express interest in the beta. Gamers will be added to the Closed Beta as the service ramps up."

http://forums.steampowered.com/forums/showthread.php?p=28770251#post28770251