How is an "actor group" different from the "global actor" support in the proposed design? Isn't this exactly a way to have a group of actors share the same queue? I gave some examples of use cases in the actor thread.
One reason I can imagine multiple objects is not necessarily sufficient is that you might have a layered library design where the a lower layer defines an actor and you also want to provide a higher layer that defines an actor implemented in terms of the actor in the lower layer without requiring all interactions to be async. For example, maybe this is a persistence library and you only need one serialization context.
As I understand it, this is roughly equivalent to the notion of a pure function, but with allowance for in-place mutation of uniquely referenced data.