Although this is usually not necessary because they will be prompted to sign on the Check-in App any unsigned documents, you can re-send the email request to sign.
Emailing link to sign
Navigate to their Member Profile page
Find the Documents section
Use the 3 dots menu and select 'Email Signature Request'
Signing in person
If you have them with you, they can either use the check-in app to sign, or you can do the following:
Navigate to their Member Profile page
Find the Documents section
Use the 3 dots menu and select 'Sign This Document'