Most of the time your users will become your contributors. That's mostly true for libraries targeted at developers, but even if you build software for non-technical users some of them will be developers and will eventually contribute.
Don't expect your community to build large features for you, even though that happens from time to time my experience is more that most people contribute small patches and improvements. That's great too because the small things can be quite time-consuming, so if your community has your back on things like documentation or translations it can help a lot.
One of our latest open-source projects is Klaro (https://github.com/kiprotect/klaro), an open-source consent manager for websites. We already got some really great contributions from our community, most people that contributed did so to solve a problem they had with the software (e.g. missing translations) or to build something they wanted for themselves. We don't accept all PRs but if they are in line with our development goals we're happy to merge them.
I wrote an Elixir mix task that would create Ecto models based on an existing database schema. A few people contributed with most being things that they would want or changes that would be minor (such as spelling mistakes or fixing documentation). The benefit I think I had in getting those contributors is that the code base was very small so it was easy to take in at one time and it was in a language that is going to attract devs that are putting in the effort to learn something new and may want to experiment with the language.
I have a small lib that I wrote to solve a specific problem at work and open sourced it, it's on my personal github profile, with the company copyright. Never made any sort of advertising or publishing on it: it was just there sitting idle on github, and it wasn't even getting commits since it just worked in the only place it was used.
One day I got a random email a person who found it through google: the name of the lib contained the keywords they were typing. I got a "thank you" email and that's it. It's been 10 years. I have no idea who else might be using it.
The technique I've usually seen succeed is when the author begins by posting an informative write-up about their project to developer focused websites (kinda like the one we're one right now). You'd ideally want to highlight the future roadmap of the project (eg. things that you want to incorporate into it) and ask people with relevant skills to contribute and/or spread the word. All the best!
I don't know, but if someone is interested, they can try. Note that I do not use GitHub; I use Fossil. Which newsgroups might I try? I did post on HN too (and got no responses), and on IRC, but I don't know which IRC channels to try. I tried to ask them but they don't know either.
Don't expect your community to build large features for you, even though that happens from time to time my experience is more that most people contribute small patches and improvements. That's great too because the small things can be quite time-consuming, so if your community has your back on things like documentation or translations it can help a lot.
One of our latest open-source projects is Klaro (https://github.com/kiprotect/klaro), an open-source consent manager for websites. We already got some really great contributions from our community, most people that contributed did so to solve a problem they had with the software (e.g. missing translations) or to build something they wanted for themselves. We don't accept all PRs but if they are in line with our development goals we're happy to merge them.