Go to Document Template record page and Click on Document Signers tab as shown below.
2.Click on 'Add New Signer Configuration' as shown in below image.
3.Please Add all the details as shown below.
Signer Email : Enter API Name of Email field from where you would like to fetch email.
Signer Contact : Enter API Name of Name field from where you would like to fetch Name.
Signer Sequence : Number of signers.
Signers query : Enter API Name of Email and Name in below query and enter it .
Select Id,Name,Email from Contact where Id = ParentId
You can add as much as signers you want.
By using this method , you can add signers in the org.