You can add notes to a contact by clicking the Notes tab in the sidebar when viewing the contact record. Here, you have the option of creating a new note or commenting on an existing one. 

To add a new note, type your message and click "Add Note". In order to comment on a note, you just need to type the information you want to share in the "Comment" field and press "Enter" on your keyboard.

Your notes and comments will be visible to your whole team.