Activity feed privacy issue

We expose a private non-joinable group in the following scenario:

- User A (Tenant X) follows User B (Tenant Y) (and User B follows User A)
- User C (Tenant Y) creates a private group (Group Z) and adds User B as a member
- User A gets an activity that says that User B has been added to Group Z

As the group is private and User A is not a member, the activity shouldn't have been delivered, as it potentially exposes sensitive data.

When fixing this, we'll want to add a test for this specific scenario