I also use the same API to get information about a meeting based on the MeetingId.
When looking at the payload there is a "Participants"-property that contains "Attendees". Problem is that this does not reflect the participants in the meeting, but rather it only holds data that was sent when the OnlineMeeting was created.
I need a way to get a list of all participants in the actual meeting even if they was not added to the OnlineMeeting when it was created (ie if someone joins using the URL or if one of the attendees that was added did not attend). There is a "Participants" option inside the Teams app that lists the current participants but this data is not the same as the data returned from the Graph API.