Reddit user j_dirty posted an image from Gamestop, stating that you are guaranteed a spot in an upcoming closed beta for Rainbow Six Siege if you pre-order. Note that this is not the same as the recently announced closed alpha.

The closed beta will be available on Xbox One, Playstation 4 and PC.

Update:

Ubisoft has officially announced the closed beta. You can enter your pre-order code or sign up for the waiting list here

Be sure to read up on the read Closed Beta FAQ and the difference between alpha and beta.