Actually, I think what you want is partitioning:
Simple parent/child aggregations are meant for getting more details about users when the list user endpoint isn’t enough. For example, the parent might be /api/listusers, which returns a simple array of users that is missing key information like email and title. If there is another endpoint to get full user information, like /api/user/{id}, then you can use the child account aggregation to get those full details.
Since you are starting from a list of groups, you can’t use the parent/child aggregation pattern since the connector doesn’t know that your parent is a group and not a child. That is why your users are getting aggregated with an array of ids.
Check out partitioning and see if that solves your problem. If you figure it out, we would appreciate if you can share your solution back here for future readers. You could also make it a blog post 