But since each investment club is unique, each club will need to decide if it has any registration requirements.Membership interests in the investment club may be securities under the Securities Act of 1933 (Securities Act).

You just didn't get caught.And god help you if you do it to a borrower in financial distress or if you do it in a state that prohibits equity skimming, requires licensure or registration, or other nefarious acts against homeowners.

You will need to work with lawyers that specialize in handling those registrations.

If I put my name on the registration forms, that defeats anonymity.

Since loans are considered securities, most states require a security exemption, which can be prohibitively expensive.At least one state, California, automatically grants licensed CA real estate brokers an exemption without requiring them to go through the expensive registration process.  

This means that you will probably need to pay registration and filing fees in at least 2 states if you don’t buy CA property as a CA resident.Be sure to tell your accountant that you may now need to file non-resident income tax returns in each state where you own property as well.

But, I wonder if being an city-approved Airbnb host with a legit registration # is enough grounds to stand on legally if an Airbnb client reports/sues me because of, I don't know, a medical injury because of hitting their head on the short ceiling (even if I disclose info about the ceiling upfront on the listing).

Are they licensed which means state MLO and NMLS registration if they are it has to be on the first page of their website by law. 

The contract must also contain the company name of the contractor and registration number.

In my lay opinion the LLCs you create to bundle your investor funds are investment contracts subject to registration.