First: the existence of the grand jury is supposed to be secret.
Second: how does Dershowitz know who is in the jury pool?
The existence is supposed to be secret, but it has been all over the news. Obviously, Mueller had it leaked. Did you listen to the news yesterday? Trump’s lawyer said he did not know about it until it was in the news. And as you say, he said it should be secret. It is very disturbing.
He does not know who is in the pool—just that it is in DC, and he can indict in DC, where the demographics are known.
I am still awaiting the grand jury ruling on Obama using the IRS to fight his political opponents.
It only reinforces the appearance that this is a political operation rather than a legal operation.