she said she mentioned in book 4 about it, but i didnt find anything about it.
Well, for those who were as clueless as me on it, she states that only the Order knows how to do it because Dumbledore taught them. Here are her reasons:
1. they are efficient because they are an anti-dark arts device
2. not hindered by physical barriers
3. each Patronus is unique to each wizard, so the Order knows who sent it, as well as no wizard can produce another wizard's patronus, so no false info can be passed...
and best of all, Rowling states that Dumbledore's patronus was...yes...a phoenix đł