Hi @crowleyb,
Great question! In order to get the registrants for a specific instance of a meeting, you will need to add the occurrence_ids query parameter to your request URL.
Note that you’re correct in that if a meeting hasn’t been held in the last 30 days, it will have expired and won’t be possible to query via Meeting ID.
Let me know if this helps!
Will
Note: This post has been edited for clarity.