New community members need to be able to get your project up and running in 20 minutes or less, otherwise they lose interest and move on
Create a 5-minute "pitch" video about your project so they know why they should be interested in learning more about it
Start by building a local community, with meetups and person-to-person networking. But you will eventually have to be online in order to scale your community.
Identify what kind of members you want in your community, then go to the places and events where those people already are.
Spend the time to find a good venue sponsor, it is worth the investment
Look for food sponsors, food is always a good draw
Invest heavily in your early contributors, they will become your core group that will help onboard other contributors later
Find wealthy (or at least commercial) consumers of your project and ask them to support it's events and activities financially
Call schools and offer to give a presentation to their classes about how to use your product. Especially open source projects, the faculty are generally quite favorable to this. Offering swag to give away to students is also a great way to get in the door with them.
Find the relevant podcasts for your project and ask to be on as a guest
Blog about everything you do, if you have the opportunity to contribute to someone else's blog or new site, take it
Write regular newsletters, post them on your blog and email them to your community
Have great documentation and code examples
Be on every social media
Create a code of conduct