Before persuading your listeners to contribute to other people's projects, please teach them how to offer their contributions constructively, respectfully, and with humility. One should find out what the project maintainers want help with (if anything), and self-assess whether one is qualified to do that work.
When someone goes in feeling entitled to have their ideas/work accepted, it can easily end in frustration on both sides, and be counterproductive to the project rather than helping it.
Also, please remind them to bring a healthy dose of patience. Even small contributions to open-source projects can sometimes take far longer to be accepted than many people expect. It might take months or years rather than days or weeks.