If you don’t know, the members API isn’t documented because it’s possible that it will change.
However, you’re more than welcome to use it. My suggestion is to manually create a member through the Admin interface with dev tools open, and you can see what data is being POSTed to what endpoint. You can also see every single validation schema here (canary=v3 at the time of writing):
https://github.com/TryGhost/Ghost/tree/master/core/server/api/canary/utils/validators/input/schemas