All PlayStation Plus subscribers worldwide can download and start the beta of Hardware: Rivals right now.
If you want to recap what the game is about, check the announcement here:
You will get a beta access key as soon as one is available after starting the game, as players will be joining the beta in waves. When you get a beta key, you also get the chance to invite one of your friends. All participants of the closed beta automatically get a key for this open beta. When exactly these waves of keys will be released, you can find on the official PlayStation forums:
JerriKoe - Game on!