Public Service Announcement time: DO NOT, we repeat, DO NOT TRY TO BUY GAME TESTING INVITES. The tragic ending to that tale is you getting your credit card information stolen and used for fell purposes. Furthermore, you will NOT get a testing invite. True story!
How can you tell if an invite is real? You will receive an email from firstname.lastname@example.org. Log into your official SWTOR account and you will be able to see the invitation at www.swtor.com/tester. Please be sure to examine emails carefully and do not click on any links until you are sure of their authenticity.
To read the official alert from BioWare, read the Dev Tracker post here.