In your Inbox you will find a Contacts section where you will be able to manage your contacts. You can add a new contact by clicking on 'Add new contact' in the top right corner, and you will see a pop-up where you can fill in the contact details. When you're done, just press 'Save'! It is currently not possible to upload these via a CSV or Excel file.
Note that the contacts in your Inbox are not the same as your contacts in your Dashboard. If you require a contact to be in both the regular dashboard as well as your Inbox Dashboard, you will need to add them twice.