Investors will be able to buy on exchanges. It doesn’t look like there will be a public ICO, and if there is, the ICO won’t be available to any US investors (retail, accredited, or institutional).
Once the token hits exchanges (I’m guessing a few months from now) - investors will be able to purchase it. My advice to my brother (this is not advice to you, use your tasty brain juice to come to your own conclusions. don’t trust anyone, not even me.) will be to dollar cost average in an offensively small amount of the course of 1+ year(s).
The reason Human is doing a soft launch now is to attract website owners to start using the hCaptcha.com and hopefully find bugs before they do their loud launch (1+ weeks from now)