This is a question I consider every day, and I wish I had a good answer. I'm not an expert on building communities. Some ideas I think work:
- Foster a community that is respectful and inclusive, especially to people with different backgrounds and experience levels.
- Call out specific tasks that new contributors might be able to tackle(ChakraCore has a your-first-pr label, for example). Offer mentorship if possible to help contributors make their first contribution as well.
- Encourage non-code contributions. Things like documentation, PR/issue templates, wiki, etc. are often more important than feature work.
- Evangelize. I've been speaking at some conferences lately about how people can get involved with ECMAScript. Sometimes directly reaching out to people is all the push they need to get involved.
Fundamentally, though, if your employer can't support you working on open source, you'll have to carve time for it. This can be hard and I don't have any good ideas to make it easier.