Ubisoft have said that you can now register your interest in Rainbow Six Siege's closed beta. Its a little vague at the moment but the R6 Seige closed beta will take place sometime later this year.
Once you have opened and registered your UPlay account you will be added to a waiting list for Siege's closed beta. So this isnt a definite way to get on the Beta. If you are looking for guaranteed beta access then the best bet is to
Pre Order a copy of Rainbow Six Siege. I suppose you could always - buy a R6 Siege pre-order. Play the Siege beta. Then, if you dont like what you find in the beta, cancel the pre-order. Get your money back.
Over last weekend a bunch of Rainbow 6 Siege alpha footage got leaked and is currently being pulled down from around the web by Ubisoft. So if you didnt get to see a sneaky peek of Siege trailer footage you may have to wait a little while longer. Although we decided to show Siege screenshots instead. So if you would like to take a look?
Siege is in closed Alpha testing at the moment, as they fine tune and iron out various bugs and work towards the larger beta test period. There is no confirmed release date for Siege at this point.