I don't think it's ever too early to open a game to people, as long as there's something there. You should definitely be careful about the numbers that you are allowing, and the prices that you are charging for accounts; you might even consider having a small group number planned (like the 10-20 that was mentioned) and have people apply to be testers so that you know they want to be actively involved in your game development (with ideas and testing) instead of looking to
play your game, these are definitely two different attitudes. Generally, when people are asked to purchase an account, they expect something to actually be there to play when they log in - not just chat rooms or message boards. I can think of a number of games that I've shelled out $5 for an account and found essentially nothing when I logged in - the thing about that is that those users will probably
never return. It's important to understand that just because the sim community convention is to call it "Alpha", it's literally an early access game. Players are very wary of Alpha games - and with good reason. I always feel like this sums it up:
And that is what you should try to avoid. It's never too early, just make sure that what you have isn't underwhelming, and that the people you invite to join are aware of what point in development your game is.
Community development is very important, but I would say user input is even more valuable. I have implemented many features that were suggested by users - ones that were good, useful, and I probably wouldn't have thought of myself. That said, a community where you are not terribly visible might lose faith in you - frequent updates, even small ones, make sure that your users know that you're still there and still working on it. If you aren't capable of this, I would suggest that you keep it in private development until it's more polished or you are. So many games are abandoned (or sold and then abandoned) that players are quick to lose faith and move onto another game; you rarely get them back after that, and if you do it will be much later in development.
I would suggest that if you are planning on letting other people into your Alpha, look into
version management, like git, if you aren't already familiar (I suggest bitbucket). You shouldn't let players into your development environment, and you shouldn't be developing in your production environment. You should have another instance at some other web address (like a subdomain, dev.yourgame.com) where you're working. Version management with git allows you to develop cleanly, rollback if you make a mistake, work with a team more easily, and develop multiple features at once without having to release anything until the feature is complete. There shouldn't be anything on a live site with players that you ask them not to interact with because it's broken - that's on you. Things like that should be either not present or disabled on a live site while you're developing.