Dead Island 2 is still a ways off but Deep Silver is starting to spill a little information about the game, even if it is inadvertently. The company recently launched a website in conjunction with Gamestop that outs the plans for a beta of the game sometime next year. They even included a FAQ on the site that gives us a little information on the beta plans.
The most notable bit of info given is that the PlayStation 4 will be host to the beta 30 days earlier than both the PC and the Xbox One. As for gaining access to the pending beta, the only sure way right now is to purchase the recently released Escape Dead Island, although I am sure that you can count on it being included as a pre-order bonus at some point in time.