It's fun (well, when you're working on one you enjoy), and there's plenty of need. If you're on Mangadex, just look for the requests teams are making as part of their headers or footers in scanlated chapters. It's how I found my way in.

Proofreaders in scanlation are basically touching up the translations to make them read well in English (tenses and idioms that don't always translate well), as well as checking for consistency between chapters in tone and spelling.