If you add the sender of an email as a contact, the email will show up in that contact's timeline automatically.
However, you might want that email to show in another contact's timeline (for example, the sender's organization.)
In that case, you click on 'Add to Timeline' and choose to search by contact
Start searching for the contact, and select from the dropdown. You can also create a new contact if it doesn't exist (for example, Adam & Smith Consulting)
The email will now show up on all added contact timelines.