Depends on what things you want to tackle. We have quite a few features being written by summer interns the Google Summer of Code and Rails Girls Summer of Code projects, so you definitely don't need to be a superstar 20-year JS veteran to contribute.
For syntax transforms: A strong understanding of current ES5 code is helpful, then just the imagination to "desugar" ESnext code into whatever the ES5 equivalent is.
Or you can work with code health. @hzoo got started with updating out code linter. Documentation is sorely needed, test cases, or even build steps. So, just the ability to read someone else's source code and understand the intention.