To sign up, you'll need an En Masse account and not much else. There are some limitations, however, chief among them a 200-gold cap, two characters per server, a level cap of 28, and max gathering and crafting skills of 150 and 100, respectively. There are also a few chat restrictions. You can see them, as well as the rest of the Discovery Edition specifics, at the official TERA website.
[Thanks to Davey for the tip!]