We spent 3 months going door to door, making demos to people I knew were working on similar projects or looking for one. We had a good reputation in the ops and systems engineering community because of our work on dotCloud over the last 6 years. Then we bootstrapped the open-source community with that initial group of 30-50 people willing to federate efforts. By the time the project was leaked on hacker news, the github repo and mailing list were already very active.

Early members of that "seed" community included engineers from Twilio, Heroku, Soundcloud, Koding, Google, Meteor, RethinkDB, Mailgun, as well as the current members of the Flynn project.