Hey @saurabh,
Pardon my mistake with the user_id vs id on the List Meeting Participants API.
Have you tried matching via email?
After testing, if the user joins the meeting with their unique join_url (returned from response of create registrant and List Meeting Registrants endpoint) then the email can be used to match GET /meetings/{meetingId}/registrants to GET /report/meetings/{meetingId}/participants or GET /past_meetings/{meetingUUID}/participants.
Can you describe your full use case so I can work on a different solution if my email suggestion does not fit?
Thanks,
Tommy