@Catherinestribs
Hope you are well.
I've only just seen your question and was wondering if you have managed to resolve your issue?
From your description, it looks like you could do a couple of things to filter Activities at a high level.
1. Adjust existing or ad new Security Roles and apply to Teams which you could add Users into these teams
2. The Permissions can be at a Team level on Activities, ensuring you only see records that belong to members of your team
3. Create System Views that filter Activities based on the User.
Question:
Do you still need to see all Activities at some point or do you always need the filter to be active on Activities?
Let me know if you would like to discuss in more detail