The software stack, including ixmp and message_ix, is developed by a single group of contributors in an integrated process. The MESSAGEix documentation contains complete guidelines <message_ix:contributing> for contributing to the code base, at: https://docs.messageix.org/en/latest/contributing.html. All of these guidelines apply to ixmp.
Contributors who have signed the contributor license agreement (CLA) for will need to separately sign the (identical) CLA for ixmp <contributor_license> when making their first pull request to ixmp.