Is there a reason why your code isn't open source? I like your mission, but it's very unclear to me why your codebase isn't open and I'd 100% be much more likely to try to help out if it were.
Volunteer-run tech and IT for non-profits aren't exactly well known for the highest of standards of engineering. The likelihood of someone accidentally leaking AWS creds or other PII seems reasonably possible without well established engineering practices.
Alternatively, it may involve proprietary code for a software vendor they work with that they don't have the rights to redistribute.
I'd be really interested in contributing but I'd hate to be contributing and then find out that this non-profit is actually some rich persons tax avoidance scheme or can be pivoted in a way to make money for someone.
Having the source code licensed as GPL or something would make this a smaller concern for me.
It is easy enough to have a volunteering agreement that catches this with clauses. Open sourcing the code won’t help with that scenario anyway.
Open source what, exactly? You point makes literally zero sense
Security is security, how you get there is entirely up to your organization. Also I'm not sure how having code access to their site proves that it isn't some rich person's tax avoidance scheme. In a way, all charities allow rich people to avoid paying taxes to some degree.
I'm not trying to be snarky, but there could be a fit for a person acting in good faith to help them accomplish this. I think open sourcing one's code is seen as easy by some, but it's actually a skill (I tend to get overwhelmed by the idea of open sourcing my projects and never end up doing it FWIW) and people experienced in doing so have something to offer.