It does not matter if you have registered games or any posts. Nor does it matter when you joined Paradox forums.
"It is an unfortunate fact that we can secure peace only by preparing for war." ~John F. Kennedy
The way i see it, its like this: pre release time of a game is called "development". Companies/teams who do that - develop games, consider this very important. On top of that usually you have PR/marketing department who (in Paradox case for example) often considers release sales very - if not the most - important part of their income. For that reason until public beta (which is a limited access working version of the game) everyone wants to keep the development versions "in-house".
When you sign up for internal beta, if youre accepted, they make you "sign" NDA (non disclosure agreement) thats supposed to be like a legal contract preventing you from releasing files and stuff without their permission. Now, technically, this isnt a real legal contract, and i really doubt they could do anything (from legal point) if you breach it (for example if youre minor, or if you live in a country that does not define this as a crime), and since this is a multimillion dollar business, on top of all that, nobody wants their competition to get hands on development versions of the game (regardless of everything).
Thats why i think it makes sense for them to accept people who have lots of posts (which usually requires years of forum activity), or people who have registered games (which requires you to buy them so that you can have tech support and stuff) so that there is at least some leverage and insurance that you wont breach the beta agreement. If you do, they ban you, and you do suffer some damage (although its arguable if it would "hurt" people or not).
I find it hard to believe that a serious company/team (for example Paradox who is both developer and publisher) would wanna do things other way - invite random people to internal beta's (public beta can be a different thing though) because thats almoust guaranteed way to have your game files shared on some torrent site almost immediately, which can (and probably does) hurt the game sales once they are out.
Naturally its possible that im wrong, so in that case disregard what i wrote.
In the case of small groups, though, such as what has been done here, saying something publicly that seriously harms the sales could very well generate a legal matter. However, the potential feedback and improvement to any product gained from product test groups far outweighs the drawbacks. FYI, companies do this all the time and have for decades.
Whether you sign a notarized piece of paper, click a few check boxes, or just say "I agree not to speak of what I see or do here," it's all legally binding.
edit: post counts and previous money spent in no way indicates any particular qualities or the reliability of a person. Those are poor yardsticks upon which to measure things.
Do you think a company based in US, for example, could do anything to someone who breaks this contract and lives in, say, Eastern Europe, in some country that does not recognize such contract in its legal system?
It has nothing to do with the amount of people being in beta, it has to do with practical fact that what is law in one country doesnt have to be law in another, and there is nothing you can do about that.
But tere's nothing wrong to try.