There is no formal R developer group (as @Adam_Black asked), and there is also no R user group (as @Vojtech_Huser suggested). In our current organization of open source software development there is a lead for each R package. Mechanisms for others to contribute are the usual: creating issues, creating pull requests, etc., and of course participating in forum discussions such as these.
I prefer to keep it ‘simple’, with a single backend. If we have something that works as ff was intended to work, it would work for everyone. If we have to write our code considering all the different implementations we would be supporting, we just made our life a lot more difficult.