A new batch of invites have been sent out for the closed beta of the Ground Forces content. It's not just hard work and bug hunting, however; the team is currently running a Winter Magic event through January 12th that can net participants special gifts, premium aircraft, and a week of premium account access.
It's important to understand that while the Ground Forces content is in closed beta, War Thunder itself is in open beta. Gaijin reports that over five million players have given the title a try so far.
[Additional source: Gaijin Entertainment press release]