So Mueller should bring in Hillary and Alexandra Chalupa for questions about Russia, Ukraine, and Macedonia.
Yes, he can subpeona pretty much anyone he desires for questioning or testimony in front of the GJ. But remember, a Grand Jury is not the same as a trial Jury. The GJ does not decide guilt, only if enough evidence exists to charge the subject with a felony. It is a very one sided proceeding where the prosecuto tries to convince the jury to indict.