On investing in individual mature companies shortly after their IPO:
it's very dangerous. First, you expose yourself to the idiosyncratic risks of the company. But more importantly, the company may be wildly mispriced. I don't know how to price companies. It's a skill of its own. If you want to learn to do it, maybe read something like http://books.google.com/books?id=WO6wd8O8dsUC&printsec=frontcover&dq=shannon+pratt&ei=fcfUR6q-F4TCyQSrxfWABA&sig=Fpqt8pGRjbLPZJ9e_QEQGFzQ7y0#PPA913,M1 (this is a 923-page book).
I rarely even invest in individual mature companies period -- the idiosyncratic risk, and just the effort of having to keep track of the company, is too much for me. I usually invest in index ETFs instead.
Now, if you can invest in a very early-stage startup, say via crowdfunding, that's different. In this case the company will probably go out of business and you'll probably lose all of your money. Because of this, the pricing of the company is mostly determined by the estimated probability that the company will fail, on the one hand, and the awesomeness of the vision if it succeeds, on the other. If the company succeeds and if the vision is awesome then i would guess that most investors would make money (although maybe less than they expect due to dilution).