Open source software projects evolve thanks to a group of volunteers that help in their development. Thus, the success of these projects depends on their ability to attract (and keep) developers. We believe the openness of a project, i.e., how easy is for a new user to actively contribute to it, can help to make a project more attractive. To explore the openness of a software project, we propose three metrics focused on: (1) the distribution of the project community, (2) the rate of acceptance of external contributions and (3) the time it takes to become an official collaborator of the project. We have adapted and applied these metrics to a subset of GitHub projects, thus giving some practical findings on their openness.