After you setup this user and connect using it to Mongo on Sisense you will be able to select connecting to some databases and you should select this one:
The in Collections look for Users and Groups tables. Due to JSON structure mongo connector can generate multiple tables for Users and Groups depending on how many nested JSON structures are within it. That should be relatively easy to go through, and then you should have a full structure of Users per Group. What is more, this can be imported inside Sisense Elasticube so you will be able to create a dashboard based on this and export it into any format.