As for legal protection, I am not sure what are you alluding to? If you mean to imply protection against scam, then I can say that this will be done via a legitimate business established in the USA etc. All details about the target apartment will be available for someone to verify where the money is going.
Yes the poster most certainly-- even if not explicitly-- alluded to protection against being defrauded and scammed. Not being sure what he was alluding to implies you may not have considered that particular risk for those who join this crowd funding initiative. Investing with a view to achieving some form of ROI is akin to providing a loan with the loan's interest taking the analogous role of an ROI. Loans here are almost always the secured and collateralized form of lending. An invitation to invest in or crowdsource funds for a property purchase is essentially asking for money for that purchase. Without some kind of protection for the investor, you would be inviting them to gamble.
So yes, several posters above mentioned legal protection of investors. I very much agree to having such an arrangement established, given the kind of amount needed for buying property in Manhattan. That protection is usually established in the form of valid collateral. Pure uncollateralized investments are rare, given the risks. I'm not sure how fractional property ownership works in terms of, say, one investor opting to liquidate his share in the investment. This might be complicated and will require contracts drawn up by a real estate lawyer.
There is a major difference between a Loan and an Equity investment. The interest rate charged on loan is priced in after the value of collateral, the credit worthiness of the borrower and term of the loan is taken into account. There is a FIXED expected return with no/minimal variance except in case of a default.
On the other hand an equity investment is an investment at risk. No collateral is posted for an equity investment and any profits generated are distributed via dividends. When someone invests in an ICO or IPO there are no guarantees on what the return might be. The only so-called assurances are of the project being undertaken or the product being developed. If you were to invest in Google 10 years ago, what sort of collateral would have justified your investment? Answer: Nothing at all. You would have invested with the team running the show and the idea behind it. It would have been an AT RISK investment.
The only investor protection I believe that is to be offered in my case is protection against fraud by the Trust so to say. Otherwise, it would be an investment with associated risks.