The key to building a good set of KPI's is think of them as a set of values that can help you understand where to dig next if something appears to be wrong. They also should be leading indicators to help you spot trouble before it becomes a problem.
So in your case if the "group" feature is a big value proposition for you then you should be tracking engagement (however you want to define - usage, enrolls, etc. - something that is of value to how you want to position yourself to investors, the market, etc.) and then the actions that a user would go through to be an "engaged consumer".
If you create a funnel from a customer perspective of how they would become an important customer then you will be able to track the actions they take and ideally be able to impact and improve the rate at which users will become important.
Hope this gives some guidance.