SQLite won't accept contributions from outside the core team; joining the core team is an involved process, as they're taking extreme measures to ensure it will continue to be recognised as a work in public domain in as many jurisdictions as humanly possible. You can still support the project by paying them for their work (or by getting your employer to do so):
My understanding was that you could absolutely contribute without joining the core team, but you do need to file some paperwork to ensure that code can remain in the public domain. The need for the legal paperwork is that public domain is not a thing in multiple countries.
TL;DR: they have a company that legally employs every single co-author, to be able to sell you a Warranty of Title, just in case your company/jurisdiction/etc has a problem with public domain.
SQLite won't accept contributions from outside the core team; joining the core team is an involved process, as they're taking extreme measures to ensure it will continue to be recognised as a work in public domain in as many jurisdictions as humanly possible. You can still support the project by paying them for their work (or by getting your employer to do so):
https://www.sqlite.org/prosupport.html