With all this testimony going on, are any of witness's allowed to discuss anything

[Princess Turandot]

2. Generally speaking...

I believe that pretty much everyone in the room in a federal grand jury hearing is sworn to secrecy except for the witnesses. (This is controlled by rule 6e under the Federal Rules of Criminal Procedure.)* However, a prosecutor could take a witness before a judge to have him/her sworn to secrecy regarding their testimony. That might also be part of any plea deal arrangement: I assume that Papadopolus character had to agree to keep his testimony under wraps prior to sentencing.

State courts vary on whether or not secrecy is automatically required of GJ witnesses.

*Here's the chapter and verse on rule 6(e)(2)(B):

(2) Secrecy.

(A) No obligation of secrecy may be imposed on any person except in accordance with Rule 6(e)(2)(B).

(B) Unless these rules provide otherwise, the following persons must not disclose a matter occurring before the grand jury:

(i) a grand juror;

(ii) an interpreter;

(iii) a court reporter;

(iv) an operator of a recording device;

(v) a person who transcribes recorded testimony;

(vi) an attorney for the government; or

(vii) a person to whom disclosure is made under Rule 6(e)(3)(A)(ii) or (iii).
....