As a project grows from a single developer to a dedicated group to a large community, what are best practices for setting up the project hierarchy to encourage high quality contributions across all aspects of the project (code, tests, documentation, graphic assets)?
I'm going to lead you to one of the projects from another question I answered, nodejs.
From their site, they have contributors (and over 300 of them!) as well as a "technical steering committee".
The Node.js project is sponsored by the Node.js Foundation and maintained by individual Collaborators. The Technical Steering Committee (TSC) membership consists of key Collaborators who have demonstrated both technical expertise critical to the ongoing maintenance and evolution of the project and a long term commitment to driving the project and community forward.
Essentially, you could create an "executive" team, with different team leaders to foster certain components of a large project, similar to nodejs.