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, 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.
Best Practice Tip:
Select "add to existing" to ensure the contact doesn't already exist. You'll still be able to add the new contact.
Once added, all of your emails to and from this person contact will appear on the contact's timeline automatically.