Titanfall. Competitive console shooter resource Beyond Entertainment reports that Battlefield 4 players who are registered with EA's Origin service are being randomly selected for the alpha.
EA's invitation encourages players to "take part in an exclusive trial for an upcoming title on Microsoft's Xbox One." The e-mail then leads Origin members to submit an alpha application here, warning that "availability to this trial is extremely limited."
Speaking to Joystiq, an alpha participant notes that players do not receive immediate access to the Titanfall test version upon registering with EA. Instead, accepted players are told to wait for further details after filling out a brief application.