Expand the email and click on the sender name. This will display a preview card with the name. If it reads Add contact, it means this person is not a contact in Karbon, so click on it.
Once you do, you can select whether you want to create a new contact or add this one to an existing one.
Tip: If you select "add to existing" and the contact doesn't exist, you'll still be able to add the new contact.
Once added, all your emails to and from this person contact will appear on the contact's timeline automatically.