When do you know when to add the Witness Signer?  I believe its possible using the "Specify Recipient" or the "Update Recipient" signing type.  So in my scenario, I send an envelope to an external Signer (outside my company) and they add a Witness who is a family member standing next to them. So the workflow would be, send to Recipient 1 (Needs to Sign), Recipient 2 (outside the company) is set as "Specify Recipient" , they then add the Name and email address after Recipient 1 signs.  Its three recipients. This could be adjusted if internal to the company as "Update Recipients". 

NOTE: Sometimes Signer types are Account plan specific.