To answer careful: Stafford has been promoting “Jah” and “Jah’s Witnesses” as names for around a decade or so.
Stafford’s reasoning for using Jah instead of Jehovah is like this. He argues that the New Testament probably used the divine name in the original, but he admits that he cannot prove this. So his position is somewhat weaker than JWs. However Stafford is convinced that the divine name is important and he points out that, even if the divine name is not restored to the New Testament as a whole, even evangelical critics must admit that the shortened form of the divine name Jah appears in Revelation 19. Since this form of the divine name is securely in the New Testament text Stafford makes his case that this is the form Christians should use.
Additionally is does also serve to position himself precisely where he is ideologically: closer to JWs than mainstream Christianity, but nevertheless distinct from JWs.
I don’t know anything about the number, composition or spread of “Jah’s Witnsses” but I find it entirely plausible that Stafford has built up a small community of like-minded people through online and local networks.
Edit: Sorry that’s right, he settled on the full title - Christian Witnesses of Jah. Not terribly elegant and a bit of a mouthful.