By buy in I mean buy into a share of the steemit currency, which I frequently liken to buying into the steemit economy (the ecosystem where steemit is used as a currency)
You can invest in 4 ways, purchase steem, purchase steem power, purchase steem dollars or invest your time. (Not investment advice)
I would suggest having a read of some posts about the different benefits and risks with each of those investments.
So to answer your question Steemit is available to anyone. All you need to do is open an account and you get a share of the future.