Grand juries are generally standing and ready to hear presentations on a regular basis. Jurors have a term of service and hear many cases, not just one.
They are given elements of potential charges and behaviors that prosecutors believe occurred by the "target," and the grand jury makes the decision such as, "If the behaviors the prosecutor asserts happened are proven, than it would be X crime, Y crime, but don't try charging with Z, the behaviors you asserted occurred wouldn't meet the criteria for Z."
Prosecutors don't have to prove their assertions, and can use evidence that would not be admissible in a criminal trial, such as hearsay. Grand jurors do not have to feel anything was proven beyond a reasonable doubt, usually have to have some percentage of majority, not a consensus, and generally don't hear the defense or counter-argument.
Jurisdictions that rely on Grand Juries always have them available, with calendars of cases to present to them.
MOO