The official announcement from Bioware leaves much to the imagination. Rumors suggest that current beta testers only have access to a limited event that will take place over the course of a few days and will only make the Sith Inquisitor and Sith Warrior classes available. Details will probably remain hard to come by, as Bioware has repeatedly posted public reminders that sharing information about the unfinished game is not allowed.
If you haven't signed up for early access to Bioware's first MMO, you're not out of luck yet. The company is accepting beta applications and it could be months before mass invites are sent out, so there is still time to sign up.