Outlook for Mac doesn't have built-in granular permission controls to prevent a user (User1) from seeing another user's (User2) contact information, manager details, or shared emails with other users (User3) within the same overview tab.
If you're using Office 365 and managing user access through Groups, ensure User1 doesn't have unnecessary permissions within the group that User2 belongs to. Permissions within Groups can control access to emails shared within the group.