The #1 thing we need are testers. There is such a wide variety of Roku devices, user settings, video formats, etc. that we simply can not test every scenario. This leads to gaps in testing where for the 2-3 of us who tested it for weeks never saw a bug that someone else experienced in the 1st hour after release.
Coding wise, it's easy to get involved.
You can pick a bug ticket and fix it.
Or pick an enhancement ticket and make it.
Or pick a feature request and make it.
Or add something you personally want to the client.
That last one is how I joined. I wanted to play music in the client, but at the time it didn't support music playback. So I wrote it.