You can't? In what jurisdiction? I dropped ten grand in a mate's (UK) startup a couple of years ago and all we needed was an accountant to formalise the transaction and issue me a share certificate.

In the US you need to be very rich to do this as you need to be an "accredited investor" which means either millions in the bank or work with a Wall Street bank to get a license to invest. Normal people are prevented from investing until most of the alpha is gone.