Developerhas that they'll be starting with a smaller beta population, and will then gradually ramp it up over a six week period. So don't worry too much if you're not in the initial batch of testers.
The Awakening is apparently 'Act 4' of Path of Exile, and digs into some of the NPC characters' back stories in the form of missions set within their dreams. As well as nine major boss fights with existing characters, the expansion will add a bunch of new gems for customising your heroes, and – of course – a bunch of new equipment and gear.
No new character class then, but still plenty to be going on with. Head over to theto register for a beta account.