It is a smart move to just send invites out without posting alot of hype about it. Less hype about beta= alot more focus on fixing issues and making the game better.
I was listening to a developer video and many times it was stated, that they have hundreds fighting. I am 95% sure that the alpha team does not consist of such number.
https://www.youtube.com/watch?v=27MjH8hi1zQ
It is called closed beta for a reason.