For most journals published by JMIR Publications, we are continuously accepting applications from senior scientists to join the editorial board. For most journals you should hold a PhD, MD or engineering degree, or similar higher degree and have a publication track record (h-index>8). Ideally you should have academic editing experience.
If you don't fulfill these requirements (yet), you can still apply to guest edit a theme issue - usually this is a great first step to gain experience as editor (see How to guest edit a theme issue).
We remunerate our editors by "karma credits" (worth up to $70-$100 per paper guided through peer-review, as well as additional honoraria for acquisition editors when they refer authors).
To apply as guest editor for a theme issue, see How to propose a theme issue.