Re: this question from my IT Director, submitting it as an idea.
As I understand it, our suggestion is that Group Joins be added to the Get User Activities endpoint.
We would also have a use case for Unsubscribe from Email Notifications (for that Group) activity.
It also might be an interim solution to this idea:
Enabling the ability to build off this activity in the API could allow those of us that need it to build notifications or workflows.