Recent server enhancements have allowed Valve to open Dota 2 to all prospective players.
For some time now Dota 2 has implemented an access system based around membership queues. Rather than opening the proverbial floodgates to new users, the game's developers at Valve have expanded its player base gradually. This was done to help it gradually improve the game's technical infrastructure so that new players would find themselves walking into a well-working experience rather than a game with lots of users struggling to keep up.
That being the case, Valve has "recently completed a set of server management upgrades as well as released a huge number of enhancements to the new user experience" and, in turn, has announced that it will be "[removing] all restrictions to playing Dota 2."
"Signups are no longer required to access the game," said Valve in a blog post. "We encourage everyone to join the 6.5 million active monthly users currently playing Dota 2." The studio also encouraged established users to use new recruitment features "to get additional bonuses for both you and your friend." In other words, if you haven't had a chance to play Dota 2 and want to, all you need to do is hit up the game on its Steam page and enjoy yourself.
Source: Dota 2