Contributions are always welcome, no matter how large or small. Before contributing, please read the code of conduct.
Much of the prelude of Babel's Contributing guide also applies here since babel-minify is just a set of babel plugins, a preset and tools built on top of these things with babel itself.
Feel free to check out the #minify channel on our Slack. You can request access here - https://slack.babeljs.io
To help develop babel-minify, check out the docs/ directory.
When you feel ready to jump into babel-minify source code, a good place to start is to look for issues tagged with help wanted and/or good first issue.