It's not something that's directly possible right now. One option is to generate a bunch of download keys that require to be claimed and have them listed somewhere people can grab them. Then you should also keep your game published, but disable new purchases. This for require someone to have a download key to get the game, and since they must be claimed to an account it will limit the max number of people who will have access.
In the future we might have an official way to do this since I can see it being useful.