In simple terms crowd funding means that a large number of individuals making investment of relatively small amount of money.
There are two questions to analyze
the first question is CAN I invest in a crowd funded investment… The answer is yes provided that you are not on both sides of the transaction… For example you are not working or personally investing in the company that is seeking the crowd funded investment
the second issue is HOW the crowd funded investment is structured
if the crowdfunding investment is structured as equity (e.g. stock in a corporation, membership interest in an LLC, etc.) the investment may be subject to unrelated business income tax if (I) the entity is an active business (e.g. providing goods or services) and (ii) the entity is NOT taxed as a C Corporation. Note: If the underlying investment is merely a passive real estate investment (e.g. long term rental) rather than a business the UBIT issue should not apply.
if the crowd funded investment is structured as debt there is no unrelated business income tax issue as long as it is TRUE debt (e.g. some debt can be converted to equity such as a convertible note)
the final issue are the standard issues with any solo 401(k) investment
must be in the name of the solo 401(k) e.g. the investment docs must be titled in the name of the solo 401(k)
the funds must flow in and out of the solo 401(k) account e.g. to fund the investment the funds will flow from the solo 401(k) account & the return of the investment will flow back to the solo 401(k) account