It's not the sort of thing that you could do easily just by amending a specific file, it would need quite a bit of work.
Basically members roles are users or admins and that's it.
My understanding is that the roadmap includes customized user groups which are based on an set of permissions. However, I think thats coming at a later date (not sure when, maybe ? 4.x.x).
There's this thread that mentions it but it's also in a few other places somewhere (ignore the versions mentioned in this topic btw):