Comments are private when the sharing settings of a post are set to "Mentors only." You can tell the post is shared with mentors only when the group name is in yellow in the post header:
Only the post creator can edit sharing settings. To make comments private:
- Navigate to the post with the comments to make private.
- Tap the three dot menu on the post.
- Select "Edit Sharing".
- Select "Mentors only" on the appropriate groups.
- Tap "Save".