How to add private notes to an employee's or client's card?

You can make private notes about all users of your account, which are hidden for everyone, except the account owner and the administrator.

To make a private note, go to the People section, choose cards of the required users, and in the mass operations menu select the "Edit" item. 

In the card editing window, click the "Contacts" button: 

At the bottom of the window, click ​“More fields +” and ​“Hidden data+”. Enter the necessary information in the field and click the "Save" button.

Here you can fill in the participant's contact information (add a phone number, as well as other contacts and pages in social networks).