The official one is https://git-wip-us.apache.org/repos/asf?p=incubator-calcite.git, however you are welcome to send pull requests via github.
The downside of github is there is no notification on new PRs, so wave your hand in the list as you create a PR or open an issue in Jira: https://issues.apache.org/jira/browse/CALCITE Vladimir
