Announced on the official forums:
So if you have any friends who haven't bought GW2 yet because they want to try out the game before buying it, get them ready for their 4-day trial period!We want you to experience Guild Wars 2 and Lost Shores with your friends, so on Monday November 12, we will turn on Refer a Friend Free Trial. You will be able to invite up to 3 Friends to play Guild Wars 2 with you for free from November 15-18.
To do so, you need a Guild Wars 2 account in good standing that was created before 23:59 PST (7:59 GMT) on November 6.
We will give you more details once the program goes live, but start thinking about who you want to invite!