When adding recipients, you have the option to have them authenticate themselves either when they are opening the sendout, signing the document, or both. To do so to a new recipient, click on the authorization tab in the top right of the Edit Recipient window. To do it with an existing contact, simply click on Authorization to the right of the recipient's role.
You have a few options for authenticating the recipient. They can either do it with a security question and answer, sending a verification code via SMS, verifying by logging into their LinkedIn account, or using their BankID. These options are available both for the opening and/or signing of the document.