Add a permission called "can see all users from my cohort" that can be turned off, to prevent a user from viewing the profiles (or finding in any other way) a user that is not part of a cohort they are in.

For example, in a course in separate groups mode, a teacher can only view participants and grades of students that are in their own group unless they have the see all groups capability enabled. Could this be implemented on a site-wide basis using cohorts or in some other way?