Here is a the list of all the sites where you can grab INVITES.
Unfortunately, these keys only allow access to the first Alpha Test from June 24th-27th. Any other tests will require their own key, which means, they are going to more chances to play.
For those who missed their chances to get in on the action, Master X Master has stated the first Alpha Test is not under NDA. That means screenshots, game plays, and any discussions about the game is open to the public. MXM will also be hosting their own gameplays on Twitch, so anyone interested can take a look at the game.
You can find more about the game at PlayMXM, the official MXM Forum, at the /r/PlayMXM subreddit, or watch at the PlayMXM Twitch.