Ask HN: How to opensource a multi-person project?

I recently worked with five other people at a Startup Weekend event where we created a product. Initially everyone was interested in continuing to work on it beyond the weekend, but after several weeks two of the people involved have decided they no longer want to be directly involved. The remaining four (myself included) are in the process of determining how best to use the current codebase that we developed. We all want to make the assignment of intellectual property clean and have all agreed that opensourcing it make sense for everyone. This allows the two people who are leaving to be able to reuse aspects of what was created (either for portfolio material or for functional elements) while allowing for the rest of us to form a company and fork the project to continue development. The project is service-based so we would not have to deal with issues of licensing the codebase in the future.

The question we have currently is how exactly to go about opensourcing the project. Do we all need to sign over our intellectual property? If so, to whom? And after that how do we make it "open source"? Does it have to be posted and made publicly available online? Or can we just attach the desired license (we were thinking MIT) to the codebase, then make sure each original member has a copy?

I understand that I am asking for advice about a legal matter and merely want information about how it could be done. I appreciate any information, resources, or accounts of how it has been done in the past.