Prior to the 2019 tax year, we used 525990 as the business code number on the 1065 form.
This number is one of many detailed by the North American Industry Classification System (https://www.census.gov/eos/www/naics/index.html), but there are many more which are used infrequently enough to not be listed on the IRS' instruction/information page for the 1065.
525990 is still a valid number, but as part of our review process, we decided to use a different classification number, moving forward.
Starting with the 1065 for tax year 2019, we began using 523910. The title for this is Miscellaneous Intermediation, but unlike the 525990 code, the 523910 code specifically mentions Investment Clubs as an example of the type of group that might use 523910 as a business code.