Privately-held stock, i.e. restricted stock of non-reporting companies, can't be sold "to the public" and generally must held for at least one year [1], but other than that is quite transferable pre-IPO. That minimum holding time can vary, too, depending on circumstances. Most transfer restrictions are put in place by companies, not law.
It's not legal to go out and raise a bunch of money from unaccredited investors. Companies will most likely have to get approved for Regulation A+ by the SEC before Indiegogo agrees to sell ownership, in which case the shares are transferable once the offering is closed.
[1] https://www.sec.gov/investor/pubs/rule144.htm