In SAM for SFDC (and indeed all other hosting systems) the addition of contacts must take place within the hosting system and then be accessed inside the SAM Planning Sheets.

For internal contacts, ie part of a selling team this comes from the User table.  External contacts will need to be added to the contacts section of your CRM system.

If a contact is not associated with the specific account that you are working with on a sheet, simple perform the search function with a partial name typed in for the contacts section to be searched for the specific contact.