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).
The usual route is to first become an author (How to become an author at JMIR) and a peer-reviewer for JMIR journals (How can I become a peer-reviewer and what are the qualifications required?).
Ideally you should have academic editing experience or guest edited journal issues.
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 compile a theme issue (with recruiting authors for it).
To apply as guest editor for a theme issue, see How to propose a theme issue.