what is the most efficient reporting solution which, allows a user to run a report that returns groups and users that are provisioned in those groups?

In the past I've generated a CSV file using a combination of looping functions, the Text from Template smart service, and the getdistinctusers() expression function.

I've also had luck populating paging grids using that same expression function and some of the functions available in the People Functions plugin.

I haven't used it before, but the application shared at this link might be helpful.
forum.appian.com/.../summary

Thanks Andrew, just what we required for our client. Appreciate your time, on the response.