Using marketing to gain club/organization/group member count

Hi! Thanks for helping us, new guys! Please forgive my not-really-perfect English, I hope it's not too painful to read. Anyways...

I work in graphic design and internet marketing for some time but until now, I used it mainly to get customers' emails, reach them on social, nurture and sell some stuff. You know the drill.

But now I'd like to learn how to use those things to get more people into an organization/group of people spending their free time working pro-bono, just for the idea of it, for others, whatever. So basically huge part of what I used at work doesn't seem to apply here: you don't show them benefits of your product because instead of selling stuff, you want them to work with you for someone/something else's gain (charity or else); you don't lure them in with discounts because they don't pay you; you don't show them how awesome job you will make for them because you want them to do it as well... and so on. I'm working on this stuff in my own free time so I should stress that there's no corporate, multi-milion budget to just spam a huge mailing list or create an ad for everyone in my country to see

So what in your opinion should a fella like me do to make more people join his non-profit organisation and work together in their free time? How to "sell" the idea and push those high-tier buttons people have in their heads that make them do something more than their regular job? How do I show them it's worth it?

Of course I'd love to learn this stuff from some sources you could provide but I assure you, you won't get any money from me by just posting links to your landing pages, sorry