Bud off a stub

One concern I'd have would be about spawning the new thread into a group where the person being quoted can't see it.  For example, without proper safeguards, a photographer could quote a model and post it in the Photographers Chat group.  It's true that someone could already do that manually, but this would make it simpler, and could therefore be more open to abuse.

And what if the post being quoted has multiple levels of people quoted in it?

This is probably an argument in favour of Midge's point, that the new thread should be in the same group as the original.