Anyone can join an open source project. You can test the GitHub version and report bugs back, create guides for users, tweak the UI, refactor things to make thing easier to understand, translate, review PRs, create release notes, do a study on what users like or dislike in the app, suggest new UI elements, maybe build a simple website for the project, structure an FAQ, organize the developers in calls or in issues, do a marketing push, organize a local install fest, present the project in conferences, etc, etc, etc.