For each signer, you can identify a Witness Signer. This is a person that has witnessed the signing or original Signer.
In the process, this is actually a extra Signer that will have to sign but at the moment the SIGN Request is created, the Witness is not known yet. The Signer must provide the details on the Witness when signing the document.
When this is done, the Witness will be notified by email that (s)he must sign as well.