Hey there.
Thanks for the clarification.
Since SonarCloud users exist independently of SonarCloud organisations (a user can belong to many organizations, for example), not all of the information is available to organization admins. An example includes tokens: tokens exist at a user-level rather than an organization-level, and an organization admin doesn’t have the right to know how many tokens a user has that span many organizations.
The Web API docs could probably use an update here to reflect that nuance.
And, GET api/users/groups would be used to get the relevant groups for each user in your organization.