Washington, DC usually has at least 5 or more Grand Juries taking testimony at any one time, with more as needed. Not all, but most are seated, supervised by Federal Marshals, some with 'generic' purposes, local crimes, major crime, both local to DC and Federal in nature, others with very specific subjects.
Grand Jury Service is fed by the pool of DC registered voters, random selects from the general city Jury Duty Pool. The Grand Jury sits for 21 weekdays, not on City or Federal Holidays, not on Saturday under normal conditions, not on Sunday. Grand Jurors may be polled by various prosecutors who's cases are heard, with the main purpose to get testimony from witnesses at the time the witness may be available. Some witnesses may be called again and again, and asked questions as a case is fleshed out, influenced by other testimony, as to the specifics asked.
If a particular Grand Jury does not have a quorum at the time when a witness is available, then another Grand Jury may witness the questioning/testimony of that witness, and then that testimony, captured by a court reporter, is read into the record of the appropriate Grand Jury who would ideally have heard the testimony. Grand Jurors may participate, and do, in the questioning of any witness before them, is up to the individual Grand Jurors, and all becomes a part of the record.
After 30 Calendar Days, Grand Jurors are usually released from service, and a new 'set' of Grand Jurors is appointed, hearing generally the continuing case, with earlier important testimony read from the record as may be needed, to inform the sitting Grand Jurors. All Grand Jurors are subject to recall by the supervising Judge, for at least one day, usually to settle questions that may arise, and their individual terms may be extended by the supervising Court.
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The point being, there are always live Grand Juries capable of taking sworn testimony from any available witness, and such testimony becomes a part of the record for the Grand Jury who eventually may be asked to VOTE on any charge.