You will need to know the syncID for the person you want to read memberships for.
The role of the person in the group.
If the membership does not have any syncID in itslearning at the time you do the request, a syncID for the membership is autogenerated and stored on the membership in itslearning. This syncID will have the format 'itsla-<GUID>'.
This allows itslearning to return memberships that were created through the UI, and that the external system would not know about.