By taking part you're entered into a contest to win actual loot like a t-shirt or a "permanent beta key." There'll be more Open House events held later. All progress will be saved.
In the game the player must design the most dangerous castle they can to try and prevent thieves from pinching your loot. Likewise you're going to be working through their castles.
All that you accomplish in the Open House will be carried over into the next event, assures Ubisoft. Anyone participating is entered into the Closed Beta list with a chance of scoring permanent access to the beta. You could always fork over money for the Double-O founder's program, which grants beta access.
The first Open House is July 2nd at 3.00pm BST to July 5th at 8.00pm BST.
To sign-up for the Open House event visit the official website.