Sandbox has nothing to do with beta games, the sandbox is for final, complete games ready for sale, that haven't earned their way out yet by being played a lot and thumbs-upped.
Supposedly the "Make" menu's Builds section is supposed to eventually be a way for developers to supply early builds to people, but that functionality isn't there yet (and at this point it's probably a pretty low priority).
Answers
Sandbox has nothing to do with beta games, the sandbox is for final, complete games ready for sale, that haven't earned their way out yet by being played a lot and thumbs-upped.
Supposedly the "Make" menu's Builds section is supposed to eventually be a way for developers to supply early builds to people, but that functionality isn't there yet (and at this point it's probably a pretty low priority).