For a university course I have developed a software application in a group of 12 students. So far we have been developing without a license (in a private repository), but now that the course is over we would like to continue working on the project under the AGPL v3 license. Since we are about to open a new (public) repository soon this seems like the right time to add it.
What steps do we need to take in order to 'change' the license of our code base? I could imagine that simply adding the license to the new repository is not enough, since this action can be done by just one team member.
"License" is really just another word for "permission" -- putting a license on your code is roughly as simple as deciding that you want to grant some permissions to other people, and then adding documentation to ensure everyone knows that you've given that permission. In order to do everything neatly with multiple contributors, and to follow the FSF's recommendations to clearly document your license grant, you should do the following:
Have every contributor give permission to use their work under the terms of the AGPLv3. For legal rigor, you could document this in writing, e.g., have every contributor make a comment in your project's bug tracker, or much more rigorously, sign and upload a legal document granting this permission.